Cold bore shot 100 fps faster?

[pcso112]

Something new for me to ponder about this 300 Whisper. After having chronic problems with very erratic velocities in the subsonic realm, I decided to have almost five inches taken off my twenty one inch barrel. The gun is built on a Remington 700 action and is intended to be a dedicated suppressed rifle with a YHM Phantom 762.

I finally found the time to try the gun out again and I still had some moderately erratic velocities while using Reloader 7. The shorter barrel has certainly tightened the velocities up a little, but still not what I would consider acceptable. I also tried AA2015 for the first time and it showed consistent velocities after the first few shots. The problem I am having is the first few cold shots from the rifle with both powders have about 100 fps faster velocities then the subsequent warm bore shots fired. This is a new thing with the shorter barrel. I am accustom to just the opposite with my other rifles.

The velocities I had with the AA2015 looked like this
1087
1047
991
990
993
1003
977
979
973
983

Now I am far from an expert on any of this stuff. Can any one help explain what I have going on?

[320pf]

pcso112,

First, what bullets weight(s) are you using?

As you have found, the 21 inch barrel was too long for sub-sonic loads. With the small powder charges the powder is completely burnt within the first ~ 14 to 16 in after about ~18 to 20 inches you start getting into barrel friction where the bullet is slowed. The amount of drag/friction is not necessarily consistent so to velocities become inconsistent.

Also the powders that you are trying (Reloader 7 and AA2015) are too slow. I would suggest that you try powders like AA#9, 2400 H110 and W296. The slowest powder that I would try is Accurate 1680 and IMR4227.

Good Luck and keep us posted.

320pf

PS. the standard deviation (stdev.) of your AA2015 load (±36 fps) is not too bad. A really good load will typically run about ±10-15 fps. An outstanding load will run under ±10 fps.. But a load the has a tight stdev. in cool weather can have a larger stdev. in warm weather.

[pcso112]

I am using SMK 220’s now. I started with 240’s but it appeared that they weren’t stabilizing in my 1:8 Hart barrel. I have some H110 on hand and report back with my results.

Can anyone explain why the first couple shots fired are faster on a cold bore? The barrel is new and has between 100 and 120 rounds fired so far. I cleaned the bore when I first got the gun and maybe once since then. The gunsmith who built the gun broke in the barrel. Could there be some type of cutting lubricant left over from the manufacture of the barrel that is leaching out? That may sound crazy but I am at a loss for explanations.

[cubflyer75]

Just guessing, I would say that the barrel is already broken in, and isn't causing your velocity swings. How far out are you seating your bullets? The closer to the lands you get, the more case volume you get. I suspect that IF you are feeding from the magazine, that it's a powder positioning issue. Your powder column is burning at different rates. Before each shot, point your barrel up and make sure all powder is sitting on the flashole. Lower slowly and shoot. Chrono a 5 shot group and see what happens...
Mike

[pcso112]

My results with 8.9gr H110 went back to slightly erratic again.

1068
1142 ???
1021
951
953
953
993
993
941
981
1074

Cubflyer my have something with his theory. The H110 load is not a full case capacity load and the 2015 that I have had my best results so far was a compressed load. I think I have a can of 4227 somewhere at the back of the shelf and I will give that a try next. I can say for certain that things have improved with the shorter barrel.

[amafrank]

Quote:

Cubflyer my have something with his theory. The H110 load is not a full case capacity load and the 2015 that I have had my best results so far was a compressed load. I think I have a can of 4227 somewhere at the back of the shelf and I will give that a try next. I can say for certain that things have improved with the shorter barrel.[/QUOTE]



I'd suggest 2400 would be a good one to try here. I made up some loads for my short 9" SIG and it had a deviation of about 9fps in 10 shots fired. The barrel was cold when I started. We tried the same load in the longer 16" barreled Rem 700 and it was about 20fps faster and once again quite consistant though we fired only 3 rds. 2400 will fill a good deal more of the case when compared with H110 so that helps a lot with the issue of powder moving around. I can't recall the exact load we used but I think it was about 9.5gr and velocity was in the 1025fps range for the SIG and 1040fps +- in the Remington.
Basically that indicates to me that as the others stated, faster powders are better for this application and bulk is helpful with consistancy.

Hope that helps
Frank