Evaluate my load and chrony data please.

[mcooper]

Hello guys, I managed to pull some free time for classes today. So of course I went to the rifle range. I went to use my new Chrony (shooting chrony brand "F1") and to test a load I have worked up for my M1a (Chrome lined 22" barrel). It is USGI brass (WRA 68 headstamp), hornady 150 grain FMJBT bullets, CCI #34 primers, and surplus IMR 4895 powder. THe chrony was 15feet from my rifle, conditions were full sun, from 3:30-5:00pm.

My reason for asking for your thoughts on my velocity data is that I got fairly good velocity from a lighter than expected powder load...but anyways, let me give you the data (FPS):

40 grain charge: 2718, 2745, 2741, 2765
41 grain charge: 2725, 2721, 2713, 2733, and 2705
42 grain charge: 2743, 2778, 2776, 2796, and 2773
43 grain charge: 2840, 2815, 2828, 2821, and 2829

I believe my target velocity should be about 2800FPS...more is higher pressure than needed (for 150grain bullets), and I'm only trying to recreate (and do a bit better in teh accuracy department) M80 ball.

I am wondering if it is possible that my chrony could have been reading faster than the bullets were actually going?

For some control speeds I shot my XD with my carry ammo (winchester LEO 9mm 124grain bondedJHP +P load, factory specs are 1180). I got: 1219, 1231, 1224, 1227 and 1228fps. Also with port Nato spec surplus I got the following velocities (from my M1a):2823, 2784, 2819, 2825, 2839, 2833, 2753, 2825, and 2797.


Thanks for the help

 

[Jim Watson]

Surplus powder might be of a little faster burning lot than commercial cannister grade. I'd load 42.5 and call it good.

I guess your chronograph could be reading high, but then what of the surplus that you want to equal? If they match, they match.

 

[Dave P]

I say that 168's oughta go between 2550 and 2650 fps for a m14. And I would keep the 150's at about 2700.

Dave

 

[Run&Shoot]

The possible factors would be: OAL, powder, primer, case, bullet, and chrony.

Are you positive all rounds were seated at the exact same OAL? If the 40 gr charged rounds were seated a bit shorter then they would create higher pressure in relation to its powder charge and thus more velocity. This seems to me to be the most likely cause.

Or, it could just be variation in the powder, but that seems like a low probability.

If it was sunny was there any chance of the sun shining directly on the chrony skyscreens? They should have shades but that might cause some variation.

Also, you could check the bullet weights to see if some are lighter than others, but Hornady is usually pretty consistent in bullet weights.

Lastly, the cases could vary in thickness of case wall. Military brass is thicker than commercial and creates more pressure for a given charge. Perhaps some of the cases in the 40 gr group had thicker cases.

If you really want to know I would load up 20 rounds in 39, 40, 41 and 42 grains each and check them again.

 

[USSR]

mcooper,

I load 42.2gr of surplus IMR4895 behind a 170gr Lapua bullet in LC Match brass for 2775fps out of my bolt rifle. The same powder charge behind a 150gr bullet in your M1A would be fine.

Don

 

[Steve C]

Your chrono's probably reading right on. Your results from shooting the 9mm was 1,225.8 fps average velocity with a std dev of 4.55 fps (an exceptional StDev for any ammo BTW). Your average velocity was less than 4% greater than factory spec which is well within a 10% +/- off average difference generally observed when shooting different pistols even with the same barrel length.

Here is how your rifle data broke out with average velocity and standard deviations:

.............Av Vel....StDev
40 grain 2742.3 19.28
41 grain 2719.4 10.81
42 grain 2773.2 19.12
43 grain 2826.6 9.40

The 43 grain load has the most consistant shot to shot velocity followed by the 41gr load and in theory they should be the most accurate of the 4. The proof is how well it shoots on paper. You could start with 41 grs and 43 grs and see how they do when shot for score in your rifles.

 

[taliv]

another possibility is that you didn't fold it down all the way flat. i know it's a long shot, but if the sensors are closer together than they should be (because it's not folded down all the way) it would give the appearance of a bullet going a bit faster than normal.