More info. Here's one for the suppressor experts. Update!

[GAgunLAWbooklet]

I'm going to do some rimfire rifle shooting tomorow, and I'll compare COLD BORE first-round accuracy with SUBSONIC AMMO at 100 yards
to groups made when the can is "warm" and the air in it is oxygen-depleted.

All ammo will be subsonic, probably the subsonic hunting hollow point made by CCI.

The silencer will be an AAC Pilot, about 15 years old.

If I have time, and if I remember, I'll try to estimate the extra noise I get from "first round pop" and maybe I'll shoot a couple of cold-bore groups without any suppressor on my muzzle, to measure any impact shift when using a can for hunting when the first shot is the one that really matters the most.

 

[C'mon man!]

I’ve noticed up to a 5% increase in velocity on my first shot supressed subsonic 300 blackout. At 100 yards with subsonic speeds, I imagine the point of impact would be a bit lower than the subsequent rounds.

 

[spencer60]

One thing you could try is to get some of the canned nitrogen used for tires (or a can of freeze spray.. also nitrogen) and pre-charge the suppressor with it for your first shot. If it really is a problem with O2 ignition that would tell the tale.

 

[Bear44]

Just got back from CMP. I decided to stretch out this load to 200 yards and discovered two things.

1) With good ballistics info, subsonic can be accurate at longer ranges.

That's a three shot group at 200 yards with heavy swirling winds. Right at 1.5 MOA. That'll do!

2) The gas in the can is still an issue. At 100 yards it doesn't really show up, but at 200 yards my point of impact shifted low / left. It was about 4 inched low and 2 inches left. If I opened the bolt and held the rifle muzzle up for about 60 seconds the gas would clear and it was right back to original POI. In fact, that low left impact in the pic may be because the can was not completely clear of gas.

 

[timbgtm]

Just got back from CMP. I decided to stretch out this load to 200 yards and discovered two things.

1) With good ballistics info, subsonic can be accurate at longer ranges.

View attachment 2083891
That's a three shot group at 200 yards with heavy swirling winds. Right at 1.5 MOA. That'll do!

2) The gas in the can is still an issue. At 100 yards it doesn't really show up, but at 200 yards my point of impact shifted low / left. It was about 4 inched low and 2 inches left. If I opened the bolt and held the rifle muzzle up for about 60 seconds the gas would clear and it was right back to original POI.

I don't think you can complain about that.

 

[Bear44]

I don't think you can complain about that.

Nope! It was much better than I was expecting. Especially in all the wind.

 

[timbgtm]

Nope! It was much better than I was expecting. Especially in all the wind.

Yep, what caliber was you shooting ?

 

[Bear44]

Yep, what caliber was you shooting ?

300 Blk. 190gr. Hornady Sub -X.

 

[timbgtm]

300 Blk. 190gr. Hornady Sub -X.

Even more impressive shooting a light caliber into the wind, holding those groupings.

 

[SOLGardener]

A couple thoughts on this- you seem very certain that it's the ambient air in the can that's causing this so try to eliminate it as an issue by spraying some computer cleaner (compressed CO2) through the can. It'll also cool it down. riskier, but you could spray butane in it and hit it with a lighter to burn it off. It's not going to be explosive, just a small jet.

I've blown a deep breath down a 9mm can to eliminate air to minimize FRP and it seems to work.

A few cc's of water as an ablative might be worth trying with subs.

Since they're subs, a thought is that you don't really have a cold bore shot since a few minutes to "clear the gas" is also enough timre for the barrel to return to ambient temps.

In that case, you could try shooting the first round at a target, next few at a different target. Then after a cool down, the next first round goes on the same target as the first round so that all of your "cold bore" shots are in the same POA.

I recall that ARmaster shooter (forgetting his name) found that the first and last round from an AR mag always had a different POI due to the bolt closing and locking.