Decreasing velocity

[Unknown]

I worked up a load using Varget, and 168 Nosler custom competition bullets for use in my TRG22, and in a 24 inch barreled DPMS LR308.

Velocity in the LR308:
2643, 2638, 3631, 2629, 2592, 2595, avg=2621fps

Velocity in the TRG22:
2597, 2599, 2587, 2613, 2544, 2534, 2572, 2587, 2565, 2578, avg=2577

I noted that the velocity for the same lot of ammo was less out of the 26 inch TRG barrel, than out of the 24 inch LR308 barrel. I also noticed that as the ten shot string continued with the TRG22, that velocity went down. I presume that is because as the barrel heats up, it expands causing less resistance, hence less velocity because of the different pressure spike.

If I take the 5 shots from the LR308, and the first 4 shots from the TRG22, the average velocity is 2599, which is about what I wanted. But for getting a true ballistic curve, should I go with the lower velocity, or the 4 shot initial velocity for the TRG22?

I am thinking if I use the rifle for long strings of fire, I should use the lower velocity, because that is what will happen. For initial cold bore shots, use a velocity of 2599fps though.

Any ideas, suggestions, or thoughts about this data? Any helpful input will be gratefully received.
Thanks.

 

[cooper_257]

Re: Decreasing velocity

I was reading an old article in rifle shooter today and they were shooting 2 rifles in the same caliber with the same load. one of them was a TRG with a 4 groove 1-11 twist barrel the other was a Lazzeroni L2000 with a 6 groove barrel. the TRG with the 4 groove barrel was 150fps slower than the 6 groove barrel. I would work up a separate lode for the TRG. a lode the TRG like might be to hot for your LR

 

[427Cobra]

Re: Decreasing velocity

Try a different Chronograph

 

[Greg Langelius *]

Re: Decreasing velocity

Note that variations in bore diameter will alter pressure curves, giving significant variations in velocity.

Generally, A tighter bore will generate a faster velocity.

While this may appear counter intuitive, the added resistance results in a higher pressure, driving the bullet faster. This is one reason why adding moly usually requires more powder to achieve the same velocity, as resistance is reduced with moly.

Greg

 

[Unknown]

Re: Decreasing velocity

Lapua says the bore in the TRG22 is a little small for caliber. I have no idea what the true bore size in thousandths is for the LR308.

I want to have one load that will shoot reasonably well in both rifles so I can avoid the possible problem of the hotter load making it into a rifle that should not shoot it. This load is really close to the Federal GMM velocity of around 2600fps, and both rifles like it for accuracy. I get a very small accuracy increase with the TRG22 if I increase the velocity, but only around a sixteenth of an inch at one hundred yards, so it isn't worth the trouble of having two loads around.

This load shoots about .75moa in the TRG, and about 1moa in the LR308, so I can live with it for the accuracy. I don't see the LR308 as a true precision rifle, I see it more like a designated marksman's rifle. So that kind of accuracy is fine.

If I raise the powder charge by .3 grains, the cases begin sticking in the LR308, and primers begin to flatten. Right now, primers aren't flattened, and the cases extract and eject just perfectly.

Which velocity would be more beneficial to use for predicting my ballistic curve?
Should I use the velocity when the bore is cold (first 5 shots 2599fps), or the velocity when the bore has heated up (2577fps)? I see advantages to using both. Perhaps compromising and using a curve between the two (2588fps) would be the best way to go.

 

[Jerkface11]

Re: Decreasing velocity

Slug the bore and find out what the real dimensions are.

 

[Unknown]

Re: Decreasing velocity

The bore dimensions don't bother me as there is nothing I can do about them in any case....they are what they are, and I'm stuck with them.

My question was regarding which velocity would be the most versatile one to use? Would the higher cold barrel velocity, the warm barrel velocity, or a compromise velocity be the most useful?

 

[ar-don71]

Re: Decreasing velocity

ive done a lot of chrono work with my 24" LR-308 and the only time ive seen a drop like that is when its cold out.if your ammo was in your car it might be warm.as it sits out it cools down and your velocity will drop.this might be what is happening.

 

[Greg Langelius *]

Re: Decreasing velocity

I can't speak for the bore diameters of specific rifles, but I agree with Unknown about not worrying when you can't change it anyway. It is what it is.

Which velocity? Whichever one corresponds to the way you do most of your shooting.

In the end, there is no substitute for taking the rifle out and wringing it out under many conditions. When you <span style="font-style: italic">know</span> your rifle, these questions already have answers. Until then, it's all conjecture, and I wouldn't be betting the farm on conjecture.

Greg

 

[Unknown]

Re: Decreasing velocity

I guess I'll just have to figure that most of my shooting is at practical rifle type rapid fire matches, and go with the lower velocity after things heat up. But, for long distance cold bore shots, I'll keep in mind that the first few shots will have just a touch more velocity.

At 100-200 yards it won't matter much. But at 600-800 yards, the difference could be a couple of inches worth of drop less. Just enough to be outside the center scoring ring.