The Montana Prairie Runner and Breaking-In Bafflement

This past week I’ve been shooting a brand-new Montana Prairie Runner .22/250, built by Montana Rifle Company. It’s an all-stainless rifle with a laminated thumbhole stock, Montana’s MRC Flats (muzzle) Brake, and a #5 contour barrel, made by MRC. The company has a break-in process that they recommend, and since I believe anything in print, I followed it.

You fire the first 10 rounds, cleaning the bore after each shot.
You then fire six 5-shot groups, cleaning after each group.

This is supposed to polish the bore, resulting in less copper fouling or no copper fouling for the life of the barrel. There’s a lot of argument about whether breaking-in works, and I’ve tried it numerous times with inconclusive results. Once before, during the process, I saw an improvement in accuracy. But what the Prairie Runner did really brought me up short.

The ammo I used was handloads, put together for my previous .22/250, which I had sent on down the road. It was 52-grain handmade bullets given me by the late Mickey Coleman, and a mild charge of R15. I didn’t care about accuracy; I just wanted something to shoot. (All the groups that follow are 5-shot.)

The first group spread itself across 2.350 inches. It was not pretty.

The second went into 2.394, four in an inch and one off in outer space.
The third measured 1.272, four in a half-inch and one out in the cosmos.
The fourth was .930, four in a cluster, one mild flier.
The fifth was .889, four in one hole and the fifth a half-inch away.

Since then, with ammo it likes, the rifle will shoot around .500, but what I’m curious about was, what caused the dramatic shrinkage over the course of those 25 break-in rounds? Jeff Sipe, who sent me the rifle, says it was the action settling in, but the action is very precisely bedded, and laminated wood is like rock and unlikely to shift or compress.

I’ve never seen anything this dramatic from any break-in process. Have any of you? Anyone got any guesses as to what changed and why? I admit I’m stumped.