Question for subsonic 450bm

[HenryTheAce]

First off I don’t reload. So I’m a complete novice.

I have a 10” 1:16tw barrel 450 bushmaster bolt action. Ammo I have is some Black Butterfly Maker Bullets 425gr subsonic buzzsaw. Originally I sighted it in suppressed (maybe shot 20-30rounds) no problem.. Fully cleaned barrel.. Just took it out on its second trip. Shot it suppressed again and it was on target 7-8 shots. Then all over the place. I mean some on target, some on target 8-10” off from POA, some not even on paper.

Went to check target and what I’m seeing is the bullet keyholing on some impacts.

Shot 7 or so full power factory Hornady FTX loads and it printed a good groups (no key holing). Went back to the subs and all over the place.

Why would it shoot good the first 30-40 shots with subs then all the sudden be way off? Is 10” barrel at 1:16tw not enough length or not enough twist to stabilize them? I notice that the barrel gets extremely copper fouled. Can easily see heavy heavy streaks of copper with the naked eye. Is it possible that they only stabilize the first 10 or so shots out of a clean barrel then due to copper fouling become unstable the more shooting you do?

Did I just get lucky in not getting a baffle strike in the beginning? Just chalk it up as a loss and not mess around with these subs anymore?

planned on using it for a 50-75yd deer gun with subs.

Bullets on the chrono were showing an average of 960fps with an ES of 120 or so.

[s4s4u]

The Berger stability calculator says you should have sufficient stability. Did you try the sub's without the suppressor to rule out baffle strike?

[plant_one]

check em at like 10 feet (without can on ). if they're keyholing there i wouldnt run them through the can... not worth the risk.

[coyote wacker]

Try cleaning the barrel and start over....I shoot the Maker 425 gr. in a Ruger American they are OK.....

Once you get the barrel clean check with a bore scope you might have a bad spot in the barrel.....