Reloading and Tuning my 16" AR-10

[Solidbuilt]

Hi,
I'm relatively new to reloading although, I've had some great success in my process, and I'm really enjoying it.
However, I just came across an "old" batch of resized .308 cases that I had planned on using for my 150gr Barnes MPG's. (multi purpose greens)... initially, I thought I'd use my comparator and set my relative headspace to 1.620... so, I went ahead and cranked out 50 cases at 1.620.... then, I put them away, and kind of set them aside for a while... I jumped over to reload for my Mini-14, Ruger American, DMR and my SPR.
Now, I'm back to my plan to reload for those MPG's.... I re-sized another batch of 100 cases, but, decided to set the bump to - .003 which lands me at 1.624/ 1.626.
I decided not to stick with the 1.620 because I had learned that a .006 shoulder bump may overwork the brass.
So, now my inventory will have some with 1.620 relative headspace and others around 1.624/ 1.626....
Should I be concerned?
I'm going to the range next week and I'm going to compare accuracy and chrono for (5) loads from each set (1.620 vs 1.626).... (150gr Barnes MPG with 45gr of VV N540).
These are for my 16" AR10 "blaster" build..... meant for extreme scenarios under 300 yards, so, I'm not too concerned about 1/2 MOA accuracy....

The reason I chose 1.620 for the MPG's is that many of my other 150gr factory .308 rounds are set at 1.620.... but, most other of my factory .308 rounds are between 1.624 to 1.626.
So, I'm going to just focus on setting my bump to -.002/.003 which lands me between 1.624/1.626 for general reloading purposes.....
although these rounds will be for my 16" AR-10, I want them to fit universally in any of my .308's....

But, as far as having a mixed batch with relative headspace ranging from 1.620 to 1.626, do you see any issues?
(I've got my (3) AR-10's pictured: 16" Wilson Combat Recon; 18" Bartlein, and 24" Shaw Bull........and, a picture of my "reloading station" on top of my wife's dresser).... the bartlein has a slightly smaller chamber and is more sensitive to hotter loads.....

 

[FLIGHT762]

So, with your comparator distance setting case shoulders @ 1.620", how does that compare to the SAAMI minimum of 308 Win @ 1.630?

What comparator are you using?

Most Comparators will NOT give you a proper SAAMI distance reading. I have 3 different Comparators. Hornady, Sinclair and Derrico. When I take a 1.630" SAAMI minimum headspace gauge and measured them in my Comparators, I get 3 different measurements.

With the Hornady, I get 1.622", the Derrico is 1.628" and the Sinclair number is way off.

So, those individual numbers are equal the 1.630" SAAMI minimum.

I use those numbers to know where SAAMI minimum is. I then can set my resized brass shoulders to where I want them.

I have a number of 308 Win. rifles. AR 10's and bolt guns. In all of my rifles, none have a fired brass lengths go past 1.634".

For me, I can set my F/L die to resize case shoulders to the SAAMI minimum (1.630") and then be able to use that resized brass in ANY of my 308 rifles without oversizing.

There are only a couple of Comparator manufacturers that have their comparators machined to match the 1.630" distance.


There are two manufacturers of case shoulder measuring tools that are called "Mics" that do measure SAAMI minimum, the RCBS Precision Mic and the Whidden Mic are set to their "0" setting to be calibrated @ 1.630". I do have a RCBS Precision Mic in 308 Win. and when I have apiece of brass resized to the "0" mic setting it is at the 1.630" minimum distance.
When I take that piece of brass and measure the shoulder with the Hornady comparator, I get 1.622" and in the Derrico, I get 1.628".

So, my question to you is that 1.620" measurement you're getting with your comparator, how does that measurement relate to the actual SAAMI 1.630" minimum measurement?

If you're actually bumping your case shoulders .010" below minimum (1.620"), you are way oversizing your brass and you will be getting case head separations.

I haven't found the need to shoulder bump shorter than SAAMI minimum, even in my two 308 Win. M-1A's.

Many new reloaders get confused with comparator numbers and try to compare and relate those numbers to actual SAAMI numbers.

I'm curious as to how you determined to set you shoulders to your 1.620" measurement.

 

[Solidbuilt]

The 1.620 measurement is a relative number using my Derraco Engineering comparator.
I measured my factory 150gr ammo with that comparator and it gave me a reading of 1.620.....
I used that number, or relative measurement, to set my die so that the reloaded brass was bumped to 1.620.
(I'm not saying the exact headspace measurement is 1.620- )
I'm just saying I bumped my fired brass down to a measurement which equalled the measurement of factory 150gr fmjs....
But, then I noticed other factory brass measured 1.624- 1.626 with my comparator.
Since I'm trying to develop a universal round that'll work in any rifle, I'm just gonna set it to 1.624 - 1.626...
My concern is having some of the reloaded ammo with a relative headspace measurement of 1.620 and other reloaded ammo with a relative headspace measurement of 1.624 - 1.626 for those resized Federal cases....
But, I'll report back after my range visit.
I do think it's interesting that Federal has different factory case sizes from their 168gr SMK'S, 150gr Fusion, and 150gr fmjs....

 

[FLIGHT762]

I've measured a bunch of Factory loaded ammo and found the numbers can be about .003" plus or minus from the 1.630"SAMMI minimum. Just as you found, Factory loaded ammo shoulder datum length does vary between manufacturers.

If you run into someone with a 1.630" GO headspace gauge, take a measurement of of it with your Derrico Comparator and note it down.

I wouldn't go out and buy a Go headspace gauge.

A few years ago, I was fortunate to have a Gunsmith friend who let me visit his shop with my comparators and let me take measurements off of his 308, 30/06 and 223 Rem. go gauges.

You should be in the ballpark with measurements off of factory ammo and not bumping case shoulder way too much.

I was just curious how you got your numbers since many new reloaders get confused with numbers off of a comparator and trying to compare it to SAAMI numbers.

 

[Solidbuilt]

I've measured a bunch of Factory loaded ammo and found the numbers can be about .003" plus or minus from the 1.630"SAMMI minimum. Just as you found, Factory loaded ammo shoulder datum length does vary between manufacturers.

If you run into someone with a 1.630" GO headspace gauge, take a measurement of of it with your Derrico Comparator and note it down.

I wouldn't go out and buy a Go headspace gauge.

A few years ago, I was fortunate to have a Gunsmith friend who let me visit his shop with my comparators and let me take measurements off of his 308, 30/06 and 223 Rem. go gauges.

You should be in the ballpark with measurements off of factory ammo and not bumping case shoulder way too much.

I was just curious how you got your numbers since many new reloaders get confused with numbers off of a comparator and trying to compare it to SAAMI numbers.

My GO gauge measures 1.625 with my comparator, so, I'll need to add .005 to get actual shoulder measurements to any brass I measure.
So, that puts one set of my reloaded ammo at 1.625 and the other lot at around 1.630.
Since I'm loading these rounds for inventory/ general purpose rounds for any .308 rifle, I was just wondering if there would be any potential issues with those shorter ones in the mix....

 

[FLIGHT762]

Just measure you fired cases from each of you rifles (I would measure 5+cases or an average) and make notes for each rifle.

My first year production SR-25 fired cases measure 1.631"(tight chamber) and my 2013 era GAP 10 fired cases measure about 1.634". Your Rifles should be in about the same neighborhood. These measurements are calculated from the comparator number length and converted to the SAMMI minimum go gauge length.

Don't be surprised if you get some higher datum length numbers on a Rifle that could be over gassed. I have a buddy that has a Les Bayer AR-10 that shoves the fired cases shoulders forward to 1.640". There's no way I'd believe that Les Bayer cut that chamber that long. The cases were from Factory Federal GMM. He gives me the cases to reload.

M1-A's & M-14's are notorious for shoving case shoulders forward on extraction. On the two M1-A's I own, I turn the gas spindle off and fire 5 rounds without the gas and then measure those cases to get a more accurate fired datum length.