Muzzle Velocity Puzzle

[Bitswap]

Nice forum, glad I stumbled on it. Perfect for my question.

I started reloading 308 using brass from spent Winchster gold mach and Varget powder with a Speer 165 gr hpbt. I have two 308's, a 700 pss (26" barrel) and a Browning BLR (18" barrel). I loaded 10 each in 0.5 grain increments from 42 to 45.5 grains.

As expected, the pss mv was always higher than the blr by about 80 fps. As fate would have it, my chrono didn't capture the shots at 45 grains for the blr. At 45.5 grains the blr mv was 105 fps higher than the pss.

I can't figure this out. Is it possible? The barrel length and amount of powerder could have something to do with it assuming the shorter barrel is still burning powder when the bullet exits the crown, but that only applies in reverse.

Any ideas on this? Maybe I got sloppy when measuring powder?

BTW, I did all the normal match prep on the cases. Reaming primer pockets and flash holes, trimming neck thickness, etc.

Thanks,
Bit

 

[R.W.Dale]

What brand chronograph were you using and how far away was it?

 

[taliv]

are those averages? do the extreme spreads for each group of 10 overlap?
i assume you shot 5 of each weight in each gun. post the actual shot velocities for the last group if you can

 

[Navy joe]

Is it possible that your chrono is getting a false signal from muzzle blast off the short barrel? That just doesn't make sense.

 

[Bitswap]

The chrono I was using is a ProChrono Digital. It was about six feet away from the rifle... hmm muzzle flash... interesting.

Here are the last five shots recorded on the chrono. I'm not entirely sure if they are all 45.5 grains, especially the first one.


Pss : Blr
2807 : 2710 << suspect blr is 45.0, not 45.5
2812 : 2861
2836 : 2892
2766 : 2818
2801 : 2962

Here are the chrono readings from 42.0 to 44.5 which are ok.

pss blr
42.00 2510 2428
42.00 2549 2473
42.00 2578 2482
42.00 2574 2464
42.00 2583 2460
42.50 2599 2515
42.50 2583 2492
42.50 2578 2501
42.50 2583 2501
42.50 2583 2506
43.00 2624 2549
43.00 2624 2510
43.00 2635 2539
43.00 2645 2559
43.00 2650 2539
43.50 2661 2578
43.50 2656 2569
43.50 2661 2569
43.50 2656 2544
43.50 2682 2588
44.00 2710 2574
44.00 2721 2588
44.00 2688 2594
44.00 2710 2588
44.00 2704 2614
44.50 2743 2609
44.50 error 2661
44.50 2710 2656
44.50 2710 2650
44.50 2726 2619

 

[jerkface11]

6 feet is WAY too close to chrony rifle or even pistol rounds. I'm pretty sure 15 for pistols and 25 for rifles is recomended. Try that and your readings should make more sense.

 

[taliv]

6 ft is closer than i'd use the chrony, but it's obviously not affecting his ability to get decent readings. most chrony's only come with 15' of cable anyway. does any chrony come with more cable?

 

[Dave P]

One rifle with a larger chamber will cause less pressure build-up and therefore less velocity.

A barrel with larger diameter lands will allow bullets to slide easier: may be higher velocity, but might be lower due to lower pressure?

 

[miko]

OK, let me "take a shot" at an explanation.

As you increase the amount of powder, the peak pressure rises. As the peak pressure rises, the powder actually burns faster - so you have the pressure peak earlier and pressure curve drop steeper. Relatively more work of pushing the bullet is done earlier in the barrel - making the shorter barrel less disadvantageous (relatively).
At the same time the friction resistance works all the way through the barrel and depends on the velocity of the bullet.

The extra 8 inches may slow down the bullet more than the the remaining pressure would accelerate it.

To make sure, could you cut 4" off your barrel to see if the bullet speeds up?

miko