Poor velocity spread in reloads for AR-10

[McGuyver]

Hi guys,

thought I'd throw this up here and see if you got any ideas what's causing this.

I've done some loads for my AR-10 and I am not at all happy with the chrony results. the ES is over 100 fps, and the SD is really poor at around 35fps. Now contrast this with other load developement I have done with my bolt action guns where I got an ES of (IIRC) around 30-40 fps and SD of 12-16 fps. Not the best in the world, but certainly better than what I see with the AR.

Both methods weigh the powder with a scale accurate to 0.1 grain (either balance bar or RCBS ChargeMaster). Both used competition seating dies.

Now here's the difference: Bolt gun loads were seated ~.010" of lands whereas the gas gun, out of necessity to fit in standard mags, is seated at just under the max coal. Also, the bolt gun is only neck sized whereas the gas gun, due to being semiauto is full length sized.

I always though that consistency in charge weight was the major factor in velocity spread, but this has got me scratching my head. Could the fact that some of the gas is being diverted to operate the bolt, lead to some of the variability I am seeing? Is this just something that is typical of gas guns that I will just have to live with, or is there something else going on here?

One other thing I noticed that does factor in here, is that the large spread in velocity is due to the fact that the velocities were creeping upward as I shot over about a 2 hour period of time. I'm kind of stumped as to why that is happening. Any helpfull insight into how I can tighten up my velocity spread with the AR would be greatly appreciated.

 

[doc76251]

Re: Poor velocity spread in reloads for AR-10

Consistency in powder charge is nice but case fill volume mitigates many wild swings in ES.

What did the rounds do on paper/steel? If they were all grouping well w/ a 100fps deviation your chrony may be to blame.

Cheers,

Doc

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

About 1 moa at 100 yards. Not the greatest I've done with other rifles. I suspected the chrony too so I shot 5 shots through a friends chrony. They were a little lower than what I was seeing at the moment, but not really any lower than what I was getting when I started about an hour and a half before.
I even measured the voltage of the 9 volt battery since the manual said lower than 7.5 volts can give wrong readings. Battery measures 8.2 volts.

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

Okay, here's a little more detail about the last time I went out with the chrony on this load:

Thursday, Feb. 9, 2012 starting ~1:30PM
AR-10(T) w/ 20" heavy stainless barrel
308 Winchester
175gr SMK
Winchester brass (twice loaded, once fired)
CCI BR2 primers
43.5 gr Varget
2.800" COAL

T = 41 ºF, Pstation = 24.41inHG, Rel.Humidity = 60% overcast
Chrony 15 ft from muzzle

1st string

1 2464
2 2532
3 2514
4 2474
5 2528

short break, then
2nd string:

1 2536
2 2542
3 2601
4 2601
5 err
6 err
7 err
8 2519
9 2492
10 2491

A break in shooting of about 20 minutes from end of last string of 10 shots.
new conditions measured about an hour after initial readings:
T = 45ºF, Pstation = 24.40inHg, Rel. humidity = 51%

This string measured from from friends chrony:
1 2498
2 2498
3 2466
4 2482
5 2480

This string measured again from my chrony:
1 2514
2 2535
3 2501
4 2502
5 2499

If I add 8 fps, 15 ft back to the muzzle on all the shots, and use data from all 23 recorded shots, I get an average of 2519 fps, ES of 137 fps, SD of 34.7 fps
I wondered if I ought to omit the data from the other Chrony since it was not measured on the same instrument and consisted of only 5 shots. However I reran the numbers, and the only thing that really changed was it caused a shift up of the average to 2527 fps, but the ES was the same, and the SD didn't change much either.

 

[LawnMM]

Re: Poor velocity spread in reloads for AR-10

Have you played with different powder charges or did you start with this because it worked in your bolt gun?

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

LawnMM,
short answer: yes, to first, no to second.

Long answer: A few weeks prior, I did the first stage of ladder testing on this gun at 300 yards. Here's the method that I am trying to follow: http://www.6mmbr.com/laddertest.html
For stage 1, I loaded up 2 rounds each in 0.5 grain increments from 39.5 to 44 grains. 43.5 grains looked to be about the middle of a good ladder group between 42 - 44 grains of Varget. I was not able to set up my chrony on that day due to constraints at the range. I don't see any point in going further in load development until I figure out why I'm getting such crappy velocity spreads.

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: doc76251</div><div class="ubbcode-body">Consistency in powder charge is nice but case fill volume mitigates many wild swings in ES.
</div></div>

Doc,

just plugged it into Quickload, and it's calculating a load density of 101.7%

edit: hmm... something wrong here. I just checked some of my loaded rounds, and I can clearly hear the powder when I shake them. Need to go back and check my inputs to Quickload...

edit2: checked all my inputs and the only thing I can think of that may be off is case capacity. I am using just the default typical case capacity and I haven't measured the actual capacity of my brass. I'm using Winchester, which I think already has higher capacity than some brass, and these are once fired brass with only a mild full length resizing done. I bet my actual case capacity is higher than the default value in Quickload...

edit3: just finished measuring the volume of one once fired piece of brass, resized same amount as others. measures 56.5 gr H2O (quickload default is 56.0 grH2O) so recalculated load density with this weight is 100.7%. Still doesn't explain why I can hear the powder shake inside loaded rounds...

 

[X-fan]

Re: Poor velocity spread in reloads for AR-10

The gun being warmer + higher velocities got me thinking.
Are you really 0.10 off the rifling?

I have done this to myself before and am wondering is it possible you made a mistake or had an equipment/bullet change and are kissing the lead ever now and then?

In my case it turned out I was a little under 2 thou off (I believe it was a slightly different run of bullets causing the issue) and once the gun got warm bullets started kissing the lead....I think not all contacted the same, which created some strange groups, yet the fliers were not out by much.
I would clean the gun (it would cool) and I would proceed to shoot 5 bugholes or so. Shortly after that things went sideways...very frustrating!
Velocity was jumping 100-125 fps.

Was a real pain to track down, but I spun the micrometer seater 5 thou deeper and the bugholes immediately returned.

Hope that helps!

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

I'm not sure where you got the 0.100" off lands from. I never stated that. I did say that I had seated my bolt gun rounds (in a different rifle) 0.010" off the lands, but that was just to illustrate some of the differences in the methods used to load rounds for the two guns. That gun is chambered in 300 WM, whereas the AR-10 is a 308 Win. On the AR-10, I am seating them COAL of 2.800-2.795" I have no idea how far off the lands it is since I haven't measure the throat length. Didn't really see the point since I was loading to fit in a standard mag on the AR-10. I've got an RCBS mic for that, maybe I'll measure that later... Thanks for the suggestion.

P.S. when you say "kissing the lead" do you mean the lands of the rifling? Just wanted to make sure I understood correctly what you are saying...

 

[X-fan]

Re: Poor velocity spread in reloads for AR-10

Sorry I missed that little detail. I saw 10 thou and assumed.

Yes, I was referring to the bullet touching the rifling.

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

It looks like with a COAL of ~2.800 - 2.795" I am running around 0.096 - 0.087" off the lands. So the theory about "expanding to touch the lands on heating up" probably isn't the problem...

 

[okiefired]

Re: Poor velocity spread in reloads for AR-10

Yeah, I assure you (even after getting hot) you're not touching the lands at 2.800 in that "T". You should be able to set them a little further out in the AR10 mags if you want to try it (my factory Armalite mags measure 2.885 inside) but doubt you'll get them real close to the lands in a factory barreled rifle without them hanging up in the mag. Whats your neck tension and is it consistant? I'm assuming you've checked your Chargemaster's accuracy. Good luck.

okie

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

I go through the calibration routine on the charge master every time I use it. I also let it warm up for 30 min. to an hour before using to let it stabilize. On the. 308 loads I'm just using the standard carbide full length sizing die I got from Dillon, so I don't really have control over the neck tension like I do with the bushings on the neck sizing die on the 300 WM.

 

[JimGnitecki]

Re: Poor velocity spread in reloads for AR-10

You just for the first time said "Dillon".

Are you loading these rounds on a Dillon progressive press?

If so, have you checked to ensure that ALL your primers are seated fully?

A Dillon progressive depends upon a very consistent hand motion on the UPLIFT of the handle to seat the primers, and that is not intuitive to most people, especially if you load QUICKLY. If you are not being completely consistent, it is very easy for the primers to protrude or crush, with a difference of .010" in position being easy to get (I just saw that on a friend's 338 primed cases). If that happens, you not only get inconsistent ignition, but also inconsistent cartridge position in the chamber and inconsistent headspace, as a protruding primer affects both.

Primers should be seated .003" to .006" below the surface of the case base.

Jim G

 

[McGuyver]

Re: Poor velocity spread in reloads for AR-10

Hi Jim, yes I am using a Dillon RL 550B to do all my loading. However I run it like a single stage when doing "precision" loads and I run it slow and deliberate. Yes I've noticed it can be tricky seating the primers with this set up and I always check to make sure they're flush, some times giving them a push twice to fully seat them. I would think that if this were the problem it would show on my 300 WM loads too, so that's probably not it. Thank for the reminder though.