LAP and CROWN
The muzzle end of the bore is the most critical factor in accuracy!
You have polished and conditioned the rifling in the bore to the best you can hope to achieve with it but the slightest nick in the crown will spray your bullets all over the place!
At the precise instant where the bullet disengages from the rifling, there is a huge high pressure volume of expanding gas behind it from the burning powder. As that gas begins to exit the muzzle behind the bullet, it must exit in a perfectly even and concentric circle.
Look up “smoke rings” and smoke ring generators on the web and watch the videos demonstrating how the smoke ring is formed. Note that it should be a perfect donut ring rolling back in on itself. The gas exiting behind your bullet must behave in similar fashion!
The slightest nick or ding in the crown can disrupt this gas flow leading to turbulence.
At the point of the turbulence, the gas will exert more pressure on one side of the tail end of the bullet causing it to yaw in flight. Yawing bullets do not fly true!
When analyzing a spready pattern from a gun that appears to be spraying all over the place with no consistency, it usually points to a problem with the lap and crown.
As compared to vertical or horizontal stringing with a consistent and repeatable pattern, crown and lap problems appear totally random. “Flyers” can frequently be attributed to crown and lap problems when the bullet gets tipped of course as it exits the barrel.
Most milsurp rifles will exhibit some form of crown damage from handling, use and storage.
Even new rifles can accumulate slight buildup of copper and lead fouling deposits at the critical lap juncture sufficient to cause “flyers” and spready groups.
It is important to very closely inspect this area with a strong light and a high magnification lens strong enough to observe even the smallest nicks and scratches.
If the imperfections and dings are small enough, they can easily be polished out without resorting to a re-crown job. Crown cutter kits are an expensive investment if you are not doing a lot of them, will still require polishing, and extremely expensive if performed by a gunsmith.
Polishing is usually sufficient to return the gun to accuracy if you pay attention to detail.
Step #1:
Closely inspect the crown and highlight any imperfections with a dry erase marker. When lightly rubbed off, it will leave visible traces behind in any nicks and dings.
The outer surfaces of the crown can be successfully polished and imperfections ground out using a Dremel with various grit compound ROUND wheels. The trick is matching the angle of the crown with the rotating wheel while rotating the barrel or Dremel so all areas are buffed evenly.
This is a little more difficult to do with a 45 degree target crown and you may have to alter a Dremel buffing wheel to the correct angle to match.
Step #2:
Continue to buff and inspect with each grit until imperfections have been buffed out and are no longer visible. Progress to finer grits up to final buffing with very fine red jewelers rouge (the small red Dremel container). For final mirror bright polishing, add a few drops of honing oil to the compound in the container and spin the buffing wheel in it to apply to the buffer. It should have a thin and oily consistency. This will remove bluing from the crown area. It is of no consequence to leave it “bare” and apply a thin dab of oil with your finger tip when done or apply MolyFusion which will prevent rust. The crown should be perfectly mirror bright all the way to the outer edges where it rounds over to the barrel shank.
Note that for deep or severe nicks and gouges, it may be necessary to round the end of a wooden dowel to the matching angle and use it to twirl a small piece of fine grit sandpaper. Common hardware items, such as an “eye hook” will often fit correctly over the outer surface. Being rounded, the eye hook can be coated with compound and used to polish the “round over” outer diameter.
Lapping
Here is the “secret” and most important part!
The critical juncture where rifling ends and begins the crown is the most important point.
This transition area must be perfectly even and concentric.
DO NOT, as is often suggested, perform this with a brass screw from the hardware store or any type of power tool or drill! It takes a very minimal amount of time and effort, by hand, with the right tool and without risk of causing extensive damage that will require re-crowning.
Order several caliber specific lapping tools from
http://midwayusa.com/
Just search LAPPING on the site and it will take you to the page.
These are only $7.99 so buy a couple and discard them when worn.
The brass lapping tool is important for two reasons.
It is a rounded ball, unlike a screw which may not be perfectly concentric.
Soft brass is necessary so that the compound works into and adheres to the polishing surface of the tool. Without the proper softness in the brass, the compound will not adhere and “work” the mating surfaces.
Step #1:
Purchase a small tube of valve grinding compound at the local auto supply store.
Wipe the crown area with a patch soaked in carb cleaner.
Apply a small dab of compound to the ball of the tool and spread it evenly around.
Holding the tool between thumb and forefinger, twirl the tool back and forth for twenty or thirty seconds while rotating the angle with your hand so that it is constantly changing and not wearing an uneven groove in the ball head. Utilize the entire surface of the ball.
Wipe the area clean with a patch soaked in carb cleaner and remove all compound.
You should see a very narrow gray “ring” starting to appear between the rifling and the highly polished crown.
Repeat once or twice until you have a very distinct and even gray “ring” roughly 0.75-1mm wide.
Wipe the crown and the tool clean with patches soaked in carb cleaner to get off all rough grit compound.
Step #2:
Using the fine red compound, thin with a little honing oil and smear a dab on the ball of the tool.
Repeat the procedure until the “ring” comes up mirror bright.
Invert the barrel and spray liberally from the receiver end with carb cleaner.
Patch from the receiver end with patches soaked in carb cleaner, discarding as they exit the muzzle so as not to draw it back into the bore, until all compound has been removed.
Coat the lap and crown with oil or MolyFusion.
Lapping is such a critical component of accuracy that every gun I shoot gets lapped!
I have had this “cure” several very sporadic shooting and well worn milsurps and halve group sizes or eliminate “flyers”.
We carry the tool and the compounds in our range box and have often done a quick lap job on someone’s poor shooting rifle with immediate and dramatic improvements!
It may need to be repeated!
Keep an eye on your lap and crown and whenever the “ring” is no longer mirror bright, give it a little touch up with the lapping tool! Don’t forget to inspect for dings and scratches whenever the muzzle comes into contact with something while handling or shooting.
Note that lapping also works with counter bores and muzzle brakes!
For any removable muzzle brake, lap the barrel where the rifling ends as well as the inside and outside of the brake.
For a rifle with a counter bore, find a tool small enough to reach down inside the counter bore and do that as well as the muzzle end at the crown.
Pic: Lapping Tool
Pic: crown with dings before polishing
Pic: crown after polishing