A "sporterized" K31 with the stock and hand guard cut off as well as some barrel. Sights are Swiss Products with 1/4 MOA clicks.
See how it shoots. Without the up pressure on the barrel provided by a ledge at the end of the K31 stock channel, harmonics will be off. I shoot several K31's (with unmolested barrels) in after-market stocks. I use Swiss Products clamp-on muzzle brakes as harmonic mass dampers, which purpose they serve adequately.

Since your K31 barrel was cut off behind the original front sight, the clamp-on Swiss Products brake will not fit. If you have sloppy grouping, a solution would be to thread the muzzle with the tool Swiss Products sell and screw on the threaded muzzle brake.

 

Thanks.
Pretty technical reply.
When you say "sporterized", do you mean by some individual civilian?
I was wondering if this was done this way for some military competitions.
I has some import marking that has the US importer and a Swiss company in the legend.

I just wonder what the provenance is.
It's a very clean alteration and I wonder who did it and for what purpose

 

Thanks.
Pretty technical reply.
When you say "sporterized", do you mean by some individual civilian?
I was wondering if this was done this way for some military competitions.
I has some import marking that has the US importer and a Swiss company in the legend.

I just wonder what the provenance is.
It's a very clean alteration and I wonder who did it and for what purpose

Please elaborate on "...Swiss company in the legend."
Is there a Swiss gunsmith's marking on it?

At least you have a top-quality Swiss Products diopter on it.

As I mentioned, you need to shoot it from a rest and see what sort of grouping you get.

 

For comparison, one of my K31's in a Mickey MacGuinness stock. The original stock was warped. Note the Swiss Products muzzle brake, which serves also as a mass harmonic damper on the fully free-floating barrel.

My other K31 in an after-market stock, also with a fully free-floating barrel. This is a Bluegrass maple stock.

This is my diopter shooter, with Swiss Products sights. The Swiss Products muzzle brake is the screw-on version. This rifle had a chewed up muzzle, most likely from bayonet drill, and the camp-on brake would not fit.