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  #1  
Old 10-08-2009, 8:43 PM
Ahhnother8 Ahhnother8 is offline
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Default Subsonic .22 ammo going supersonic - unpossible!!

The last thread where this was discussed took a 90 degree turn AND had a title change to match. Have never seen that before.

I shot smallbore rifle competitively for a number of years. It was mostly indoors (gallery) and during the winter months. What we were discussing in the thread was that after cleaning the barrel, the first shot always went out supersonic while shooting subsonic ammo. Don't know how or why, it just does.

Was at the range today, testing centerfire ammo over the chrono. So, I dug the .22 out of the safe and scrounged up some ammo. The gun is an Anschutz 1913 Supermatch. The only ammo I could find was SK Jagd Match and Federal Gold Medal Match 900. I used to shoot the 900B, which was subsonic and Wolf Match Extra which was subsonic too. They both acted the same as the SK Jagd. Anyhow, the SK Match and Fed 900 was all I had to work with today. All numbers are feet per second.

I cleaned then fired five shots, then cleaned and fired five shots, then cleaned and fired five shots again, with the SK. Cleaning consisted of pulling a bore-snake through twice. Here are the velocities 10' from the muzzle:

clean barrel: 1113, 1108, 1113

second shot: 1035, 1028, 1031

next three "3 shot groups": 1043, 1045, 1043, 1042, 1042, 1025, 1046, 1051, 1046

At 68 degrees with dry air, the speed of sound is 1125 fps.
At 59 degrees with dry air, the speed of sound is 1116 fps.
At 50 degrees with dry air, the speed of sound is 1107 fps.
At 41 degrees with dry air, the speed of sound is 1097 fps.

It's easy to see that the velocities with a clean barrel are supersonic at anything below about 50 degrees. I also measured the velocities at 10' from the muzzle, don't know how much the bullets slow down in that 10'. The Federal 900 is supersonic ammo averaging 1164 fps. I cleaned and fired one of those rounds and it went out at 1228 fps. Also a huge speed increase with the clean barrel.

Did one other controversial test with the .22, just to confirm what we were seeing on the target and on the chrono. This also applies to centerfire to some extent. Smoke in the barrel from the last round affects velocity!! It also affects bullet impact on the target. The shorter the range, the less velocity variations affect things. I fired 5 shots consecutively - ejecting one and immediately loading the next. I then fired 5 shots but ejected the round and blew the smoke out of the barrel before loading the next round for each of the 5 shots.

Consecutive shots fps: 1046, 1045, 1050, 1046, 1027 - fairly consistent, except for the last shot.
Blowing smoke out between shots: 1027, 1060, 1034, 1054, 1057 - all over the map.

Any commentary or further insight?

Lane
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Old 10-08-2009, 9:09 PM
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Wouldn't you need know the temperature when you were shooting and the speed of sound at that particular temperature?
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Old 10-09-2009, 7:45 AM
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I notice pretty much the same thing when I chrono WolfMT from my Savage MKII. I don't really clean my barrels but when I run a boresnake through, that first shot (fouling shot) will consistently be about 40-50fps higher (~1100fps) than the others. After I run the boresnake my barrel takes about 10-15rds before it will shoot back in and I trust it.

I'm thinking that running the boresnake gets the particulate fouling out but leaves the residual lube in the bore.

Therefore: My hypothesis (WAG) is that first round has less friction going down the bore, since it doesn't have any fouling to push out and the bore has some lube left. After that first round, the powder fouling is there and after 10-15rds the fouling/lube reach some equilibrium.
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Old 10-09-2009, 10:23 AM
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Quote:
Originally Posted by Ahhnother8 View Post
Smoke in the barrel from the last round affects velocity!! It also affects bullet impact on the target. The shorter the range, the less velocity variations affect things. I fired 5 shots consecutively - ejecting one and immediately loading the next. I then fired 5 shots but ejected the round and blew the smoke out of the barrel before loading the next round for each of the 5 shots.

Consecutive shots fps: 1046, 1045, 1050, 1046, 1027 - fairly consistent, except for the last shot.
Blowing smoke out between shots: 1027, 1060, 1034, 1054, 1057 - all over the map.

Any commentary or further insight?
Long live the semi-auto brotherhood!
The semi auto action does a better job of keeping the smoke in the barrel due to it's consistent cycling.
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Old 10-09-2009, 5:45 PM
JHermsen JHermsen is offline
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My thought is you're getting various levels of moisture and temperature variations by blowing the barrel out, at least with lung power. I'll bet moisture has more effect than "smoke" particles.

My two cents.
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Old 10-09-2009, 8:27 PM
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Originally Posted by JHermsen View Post
My thought is you're getting various levels of moisture and temperature variations by blowing the barrel out, at least with lung power. I'll bet moisture has more effect than "smoke" particles.

My two cents.
That's what I would guess too.
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