This is a bit like the study of quantum physics, and what happened in the first few nanoseconds of the Big Bang.
Would this have an effect on the consistency of the muzzle velocity only, as a function of the bullet's ability to provide an effective gas seal?
From what I've read, boat tail bullets have less drag and therefore perform better at longer ranges, but with flat base bullets the extra drag at the rear helps them to stabilise quicker and therefore perform better at shorter ranges (eg 200-300m). The physics of this makes sense to me.
I wish Hornady would make a .312" 174GR boat tail FMJ bullet instead of the .3105" which is sold here.

 

This article from the Sporting Shooter of 1 February 2012 has interesting information on the effect of ammunition velocity inconsistency, and the phenomenon of compensation.

"Amongst modern target shooters there is a bunch of skeptics who, like holocaust deniers, refute as balderdash the phenomenon of compensation. In the case of the SMLE, compensation refers to the way muzzle jump variance caused by ammunition velocity inconsistency causes the elevation group to converge at longer ranges. In other words, they seem to group tighter at longer ranges than short.

Imagine that your muzzle rises through recoil (jump) and the faster bullets exit at a point lower in that jump, with slower bullets naturally exiting higher in the jump cycle. At some point downrange, the faster, flatter shooting bullets will converge with the slower, more trajectory-challenged bullets.

In the British War Office Textbook of Small Arms published in 1929, this is explained in greater length and that is where the diagram accompanying this story came from. Anybody who shoots old military rifles seriously should get a copy as it's still in print. It goes on to explain that at the Bisley meeting in 1920. Standard Mk VII ammunition was quite vertically erratic at 200 yards, while at 600 yards, the groups were much better in proportion to the distance.

Comparisons by target shooters through the 20th Century determined that the phenomenon of compensation was real, with the SMLE No 1 MkIII and Mk VII ammunition shooting more tightly between 900 and 1,000 yards. With the Long Lee Enfield (commonly named the Long Tom) and Mk VI ammunition, it compensated best at about 1500 yards. The later No4 rifles, used by the British in WW2, compensated between 400 and 500 yards.

Now I have personally experienced this when competing in the old McIntosh .303 Matches, which used to be held over two days at ANZAC Rifle Range, Malabar. A staunch band of 10 or 12 .303 shooters used to try and mix it with state-of-the-art .308 and .223 target rifles at ranges from 300-800 metres. Over the three matches that I shot, my 800 metre scores were uniformly (much) higher than my shorter range scores and in variable winds, we seemed not to be disadvantage much, if at all, by shooting against .308s.

Now I was fortunate enough to attend the Long Range Precision Rifle Shooting course in Canberra a while back and my scoped No4 Savage .303 with handloads was as able to hit stationery clay targets out at 800 yards with as much ease as modern tactical rifles. On another occasion, at Lithgow Rifle Club, I blew a good centre count possible at 800 yards with my No1 .303 on the last shot (an inner just out), probably because I could not believe it was happening. Bernie Doohan is my witness. The elevation group, off the elbows, was about 1.2 MoA for ten shots.

The ironic thing about this from my perspective is that I argue that this phenomenon is really there with my mate Mark Adamson repeatedly and he staunchly refuses to believe in it. Here's the rub - he exerienced his best scores at 700 and 800 metres just like Daniel Cotterill and I did, when fighting it out for the medals. Go figure.

Is it any wonder some of us just love these old clunkers?

Marcus O'Dean
Editor"

 

This same subject comes up with Siamese 8x50 and Carcano 6.5x52. Good post by Swede. I was not aware of the glazeboard wads. Can we make or buy those? Or learn a little more, please.

 

The original topic addresses the issue of bullet set up, or upset of the base of jacketed bullets. I do not believe such an effect exists. As mentioned earlier, bullets recovered after firing through normal-length barrels, or even those substantially shortened, show no evidence of expansion of the base, and my experience in examining a very large number of recovered bullets of various calibers is that there is no such expansion. On the other hand, those who support the theory that such expansion occurs depend on the appearance of bullets fired through extremely short barrels, so short, indeed, that the bullet extends past the 'muzzle' before firing. In such conditions, the bullet is, in fact, expanded: indeed, mushroomed. But in every case where I have seen photographs of such 'expanded' jacketed bullets, the exposed lead core shows clear evidence of impact from powder granules: the bases have been peened by high velocity impact of unburned powder ejected like shot, not by gas pressure. If the expansion occurred due to gas pressure, as claimed, the expansion would be evident and measurable on bullets fired through full-length barrels, and that is not the case. I believe that such 'experiments' under abnormal conditions , and purporting to show the effect exists, are unscientific, and that the interpretation of the results has ignored the obvious evidence for the true causation of the effect achieved. Best accuracy is delivered by full groove-diameter jacketed bullets.

PRD1 - mhb - MIke

 

Keep in mind these are Mil Surp bullets with the lead core exposed at the base.

 

This would also explain two other apparently unrelated things.
Key-holing of boat tails in some 2-groove barrels & why the (in)famous MkVIII bullet was a rebated boat tail.

 

So what are the open based boat tailed not rebated projectiles then. They were in Mk8 marked cases. I Have a die to turn them into Mk7s.
I also have Mk7 ammo with no glazeboard the cordite appears to be in a bag .

 

I regret I have no data on Turkimenistinian Khyber pass loads to offer!
My bad.

 

Plonker,
the two groove barrels (wrt obturation) should be notionally better since the actual area the bullet had to seal was reduced when the the other grooves were deleted (the 1929 textbook I have does not of course discuss this WW2 alternative barrel).

 

Just a passing grenade into the punch bowl, with focus on 1000 yd accuracy. From my Long Branch No4Mk1* with PH target sights, rifle in 99.9% condition, 2 groove bbl. The Hornady .3105 dia. 174gr BTHP are horrible, they are great out of a minty M39 with a tighter bore. The Sierra .311 dia. 174gr BTHP are superb but they truly suck swamp water in accuracy when compared to the Hornady .312 dia. 174 gr BTHP. Those .312 were a special order from Graffs reloading that Hornady produced and latest is : its not available anymore. Thus...if you like them, gobble up all you can find.

That all said, my near exhausted supply of South African 1980's era Mark VII issue 303 ammo shot better than the .311 Sierra from 1000 yd line. The SA ammo was not cordite, obviously well made for issue 303 ball ammo. If I could find SA 303 caliber 174 gr FMJFB bullets as used in their Mark VII ammo, I'd buy a few 5 gallon buckets ! Those are great bullets.

 

I wonder if Hornady are aware how crap their .3105" bullets perform in Enfields, and how many .312 bullets they'd sell if they started making them again. I'd buy a few thousand to start with.

 

It is my understanding that Hornady made .312" 174 gr bullets only for Graf's. I've used these bullets in my 1948 SAN No 4 Mk 1 dot and they definitely group better than any .311" bullet. My SAN No 4 has a pristine 5-groove barrel; it evidently was only used for parades or guard mount so bore wear is not a factor. But bore "fit" certainly is.

I've talked with a tech at Hornady and was told Graf's did not order another batch of these bullets, so ping Graf's about them if you want them to reorder the Hornady 3130G bullet. Note the "G" suffix; the Hornady 3130 - no "G" suffix - is a RNSP flat base .312" bullet.

BTW "Milprileb" tipped me off about this bullet; many thanks!

 

fourbore,
from memory the glazeboard wads are very hard. A piece of paper card at least as thick as a birthday card, coated with what looks like a heavy dark varnish which is shiny. Hard to think what modern alternative to reload with, most shotgunners would be using a plasticised milk carton or tetra-pack, but that stuff is quite thin. It would also depend on what powder level you are running as to how well you can seat the wad.

 

Swede, from post #1 I am curious how the bullet base expanded to .350". The CIP dimension of the chamber neck is .344" reducing to .340" where the throat starts. If this is correct then the only way that the base expands to .350" is if the neck and/or throat have been expanded.

Any bullet that passes through a barrel should not have a diameter larger than the groove (edited) diameter; after all, this is how you slug a barrel.

I back PRD1 in post #5 and the excess mushrooming happens as the bullet leaves the barrel.

I bought my No4 earlier this year after some research. This led me to believe that 2 groove barrels had larger groove diameters that 5 groove ones. When the bullet is set into the rifling the 2 groove barrels would have a smaller cross sectional area due to missing 3 grooves. This would lead to higher pressures. To alleviate this, I read that 2 groove barrels were given deeper grooves.

This would fit with Milprileb's post #11 where 2 groove barrels shoot better with .312" than .311"s.

I started target rifle shooting in 1977 with a No4 and all my generation know about positive compensation. Maybe it is a generational thing.

Of course it all depends on your bullet velocity. This is why you carry out a ladder test to find accuracy nodes for your rifle!

I am happy now to be flamed where required.

(apologies for the full stops; it is also a generational thing!)

 

Any bullet that passes through a barrel should not have a diameter larger than the bore diameter; after all, this is how you slug a barrel.

I bought my No4 earlier this year after some research. This led me to believe that 2 groove barrels had larger groove diameters that 5 groove ones. When the bullet is set into the rifling the 2 groove barrels would have a smaller cross sectional area due to missing 3 grooves. This would lead to higher pressures. To alleviate this, I read that 2 groove barrels were given deeper grooves.

The largest diameter of a fired bullet should be no greater than the groove diameter of the barrel, not the bore diameter. In measuring a rifle barrel to determine proper bullet fit, it is the groove diameter which must be determined. It is relatively easy to determine the groove diameter of a 2-groove barrel by direct measurement - a good dial caliper can give this dimension within .001" with careful measurement. Though it is more difficult to get an accurate measurement of groove diameter in barrels with odd numbers of grooves, such as the L-E 5-groove type, it can still be done accurately enough for the purpose by measuring groove diameter with the dial caliper against the driving edge of one land and the trailing edge of the land which lies directly across the bore.

PRD1 - mhb - MIke

 

Yes, meant groove diameter. Typo and I have edited the earlier post accordingly.

 

dave and PRD, I think the reason why the bullet was able to expand to 0.350" at the base might be the 3" barrel length. I assume they treat the usual meaning of barrel length as from the breechface. I don't have a 3" barrel, but coincidentally do have a gauging tool made from the stub of a H Barrel (the best quick test to prove your reloads are good to go for rapid fire competitions) which might be very close to that length.

My first guess was a 3" barrel has little rifling left, perhaps only the leed, so that the dimensions for the bore you discuss in these last couple of posts may not apply. Ideally to prove it we would need a sectioned barrel which a Mk VII cartridge could sit in - the answer would be obvious then.

I got this little tool out and had a closer look. Having no Mk VII handy I used a factory HXP round which has COAL = 77.2 mm. The tool is a little under 3" (76.2mm) long from what used to be the breechface, so that the HXP just sticks out the end. From what I could see the idea of testing (to see what happens between the bullet leaving the cartridge case and before engaging the rilfing as per the OP) with a 3" length removes the rifling.

The very short test barrel they used in the Textbook fully supports the cartridge case as is normal, but the bullet jumps clear of the case without touching any rifling. The cartridge pressures rise as normal until the bullet clears the case neck when the driving gases can start to freely escape around the bullet. I would expect pressure drops rapidly from that moment and the pressure would be much lower than normal after the bullet exits the case. To me it is a sound methodology to prove what happens to a bullet before it engages the rifling.

 

Swede:

As I said earlier, the photos used to illustrate the effect showed both the 'barrel', with the nose of the bullet projecting, and the fired bullet with mushroomed base. It seemed clear to me that the full chamber and part of the rifling remained (there was some evidence of rifling marks on the bullet), and the exposed lead at the base of the bullet showed very clear marks of the powder grains which had impacted and peened the base to a large diameter. Bullets fired from even severely shortened barrels which were longer than the final test case showed no evidence of powder grain impact on the exposed lead at all. Of the thousands of fired jacketed rifle bullets I've examined over more than 50 years, I have never found any with powder grain impact marks on the exposed lead at the base; nor have I ever seen or been able to measure any evidence of bullet expansion at the base to fill groove diameters larger than the original projectile. If the Textbook does include photographs of the test setup and fired bullets, perhaps some one of the members will be able to post them here.

PRD1 - mhb - MIke

 

Hi Mike, you refer to "photos used to illustrate the effect", leaving me unsure where these could be from.

In my posts I am just repeating as accurately as I can what the ToSA states. I haven't referred to photos as there are none; my 1929 copy of the textbook doesn't use any photos to show this obturation test was carried out.

 

Source document

 

Camsfirie, your'e amazing, you must have every text worth having!

The two pages I have based this post on are p209 and p269.

Being a simple soul I am happy to accept the idea .303 flat base bullet set-up occurs if the British designers of the cartridge say it does.

 

I am certain I have a reference containing the photos I've mentioned. I will find it, and attempt to post the photos and pertinent text. I am unsure in which book they are found, but believe it may be 'The Bullet's Flight from Powder to Target', by Dr. Franklin W. Mann, my copy of which is currently loaned-out. I will search other likely candidates still on-hand. It may take a while, but I will follow-up. FWIW, I make it a rule never to tell more than I actually know, and to state clearly that I am giving personal opinion or information derived from personal observation and experience when that is the case. Meanwhile, if anyone can find the information required, please do provide it.
And, for those who, like me, do not always accept received information when it appears to conflict with available evidence, I suggest measuring some fired bullets (and I'm sure most long-time riflemen have picked up and examined more than a few) and see whether any of them in relatively undamaged condition exhibit base diameters larger than the known pre-firing dimension. If such expansion occurs, as the theory suggests, there would be nothing in the bullet's passage through the bore to reduce it again to its original, smaller, diameter. Then, too, if expanded to full groove diameter, as is suggested by the theory, the bullet body would show evidence of rubbing contact with the bottom of the grooves at the base, if not further up the body. I've never found any such evidence on fired, jacketed bullets known to have been measurably smaller than groove diameter before firing - or, indeed, on bullets known to have been of full groove diameter.

PRD1 - mhb - MIke

 

I'm having a grand time following, at times trying to follow, this conversation. However I keep getting the impression certain factors are being overlooked or were overlooked by those doing the laboratory research. Then again it might be simply the conveyance within the limited parameters of text allowed on this forum.
So, not having been a physics major but having been required to take physics to graduate (along with biology and chemistry) as part of my technical curriculum, this is what I'm fathoming here so far.

From a case in an oversized chamber in relation to the bore, the projectile is suddenly pushed forward. Force begins to push at the base while the front is wanting to remain immobile for a millisecond or two.

The nature of projectile construction causes a noticible obduration effect at the rear as the case expands and the bullet base deforms filling the void left behind.

As the front of the bullet enters the leade it encounters more resistance, thus increasing the upset of the bullet at the rear.
The expanding gasses continue to push until the bullet is swaged to the smaller bore diameter as it continues down the tube.
Now we encounter friction at a grander scale, along with the unequal opposing forces of expanding gas at the rear and compression of air ahead of the bullet inside the tube.

Skip the in flight ballistics as the variables differ. From load to ambient temps and relative humidity etc.

Now we just begin to e counter terminal ballistics. The instant the tip of the bullet encounters the target. Assuming the bore is true and the bullet bucks the in flight hazards, it should strike pretty much stabilized and true on its axis.
The tip of the bullet meets the resistance sooner than the rear while momentum of its mass wants to keep pushing. Theoretically, there should be a small amount of increase in diameter gradually increasing towards the rear.

Now is where the other variables enter the equation upon recovering spent bullets.
All we would be able to find is evidence of obduration occurring at one time or another. I can't see getting any accurate measurement. Only a determination that obduration had occurred.
Secondly, when fired from a stub barrel the factors of swaging and the coefficient of friction etc wouldn't exist. The powder traces on the base only reflect the bullet didn't spend enough time in the bore to achieve full burn or enough time to gain enough velocity to measure any deformation at the terminal end. Not that there would be anything detectable worth measuring. Aside from bullet performance and weight retention and blah blah blah. All outside the real of this particular discussion IMO.

So, am I following you guys so far? Am I seeing what I perceive as irrelevant data being added to what is capable of being proven thus leading to a foggy conclusion?

I asked a lot of questions in my younger days too

 

That's why they use special traps or tanks. It decelerates the bullets arriving far more gently ( comparatively) than just something like a sand trap.
I've actual been struck by a fired .303 MkVII ball round, it had plowed through a dirt bank, penetrated a paper target, struck & bounced off a large metal number sign over the target & came to rest inside my jacket, against my shirt. I still have the burn scar!
The jacket was amazingly intact after all that (remember the "perfect" fired bullet from the Kennedy assassination?) but the jacket was completely missing. We assumed the lead had simply melted from the heat & friction.

 

That still doesn't explain how initial obduration can be reliably measured after the bullet has passed through the barrel. In all practical purposes, a swaging die.

I've seen ammo shed the jackets upon firing. A batch of Portuguese surplus sold as 7.62 NATO produced shotgun accuracy and keyholing from an M1A NM barrel.
Several yards ahead of the shooting station the ground glittered in the sunlight. All jacket fragments. Peeled away like a round ball shooting patch.
At the time I wondered if they were actually surplus CETME loadings which couldn't handle the fresh SA barrel.

 

That still doesn't explain how initial obduration can be reliably measured after the bullet has passed through the barrel. In all practical purposes, a swaging die.

I've seen ammo shed the jackets upon firing. A batch of Portuguese surplus sold as 7.62 NATO produced shotgun accuracy and keyholing from an M1A NM barrel.
Several yards ahead of the shooting station the ground glittered in the sunlight. All jacket fragments. Peeled away like a round ball shooting patch.
At the time I wondered if they were actually surplus CETME loadings which couldn't handle the fresh SA barrel.

I never said it was.
I was referring to further changes caused by bullt impach while trapping samples fired.

 

Hi folks. Here is the book to which PRD1 (Mike) refers. I'll leave the reading up to you all.
Cheers!

 

First a thanks to Mike for a fantastic reference to Mr Mann (and camsfirie for posting). I have seen numerous references to his work in later texts but never actually read this work.

I like the introduction where he correctly says the best plan must include the tests which failed to give any result. We all learn from this and hopefully avoid making the same trip down a dead end.

I attach one paragraph from page 82 and this paragraph seems to support all the ides we have collectively aired here. It addresses a lead bullet with black powder charge but it agrees with his other tests of the flat base 150 gr 30-06 load.