[jwh_Denver]

The barrel for the 50 is almost 4 feet long. This accounts for most of this accuracy.

Would it be feasible whatever the barrel length of military rifles to put threads on the end of the barrel allowing for an extension barrel to be put on for longer distances? Obviously you would want some kind of thread quard.

[Shooter 2.5]

What would be the purpose? Just mount a scope and you're good.

If you're trying for an extention of the true barrel, you would have to line up the rifling absolutely perfect or things would get very messy.

If you're thinking about a longer sight radius, blooper tubes have been used to extend the iron sight radius in competitions. The trouble starts when the harmonics start to change and the groups open up.