SASS with 1.624 headspace...go figure.

[aquinas]

Just got a New DPMS .308 SASS. So I load 50 rounds of new winchester brass with H4895, SMK 168's, CCI-BR2 Primers. Loaded in .3 grain increments for 3 rounds each from 39 to 43 grains to do a ladder test.

Before I loaded the brass, I checked each with a hornady gauge. Average headspace was 1.624 with a few down at 1.622 and a few up at 1.6265. Most were just about 1.624.

So I necksized the brass and seated bullets, figuring no need to full length size with those measurements. After all...SAAMI minimum chamber size is 1.630...right? Plenty of clearance. Uh-huh.

After shooting, I got home and checked the fired brass again with the Hornady gauge. Average size was .002 <span style="font-weight: bold"><span style="font-style: italic">SMALLER</span></span> than the brass going into the gun. Whatda???

With all measured, the average was 1.622, with some measuring as little as 1.619. A few measured 1.625.

Called DPMS and must say, the guy seemed to be giving it to me straight. Told me whoever set the headspace was probably using a worn headspace gauge. I asked incredulously if that was possible for them to wear that much and he said it was when they are being used constantly.

So I took some coffee can brass I had and sized three dummy rounds to 1.630, 1.629 and 1.627. Did not use powder. Seated a bullet in each and put them in the magazine with bolt locked and then hit the bolt release and let the bolt slam them home.

Two measured 1.624 and one was at 1.622. Used layout die on the shoulders and shoulders got hit hard.

Do you think the dummy rounds are giving an accurate indication of headspace. Why or why not?

It's going back to DPMS. Your thoughts and comments appreciated.

 

[CST]

Re: SASS with 1.624 headspace...go figure.

Based on those numbers...I dont think you should send it in quite yet.
After the chamber is cleaned...CLose a loaded round(without powder)-use a new piece of brass that hasnt been fired...and put into the chamber and let the bolt go 2/3rds of the way home and see if you can easily extract it... Having a tighter chamber is not always a bad thing as long as you get the velocities without the pressure(so you need to chrono)

I have some times noticed when you slam a round into the chamber the impact causes the shoulder to set back a hair..I dont remember how much..

I dont think you can rely on the dummy rounds to give an accurate impression.

Try using a once fired brass (from that gun) into the chamber and let the bolt slam home (make sure the chamber is clean) and try to extract the round..is it hard or does it come right out? I've had a overly tight cut chamber on one of my GAP AR10s and had to send it back.. I couldnt get the brass out without slamming the stock on the ground.

Bottom line..how does it shoot? Any pressure signs? Chrono the rounds ?

 

[aquinas]

Re: SASS with 1.624 headspace...go figure.

Yes, Yes and yes. I did a ladder test over a chrono with the gun. It was showing pressure signs at 2300 fps...but here's the weird part.

I tested 50 rounds at 200 yards in .3 grain increments of 3 rounds per increment. The first 6 shots looked normal. There were 2 fairly distinguishable groups that showed vertical dispersion . Then on the next increment, the group jumped about 8 inches (which is normal) and then the next 12 groups hung in an area of about six inches. There was no vertical dispersion.

Now someone might say, wow, that's great...you have a "node" of 3.6 grains!!! No, something is definately wrong. No gun in the world would hold six inches of vertical dispersion at 200 yards without there being a definate problem.

My velocities went from about 2250 to about 2600 fps during those six inches. 350 fps at 200 yards should disperse at least a foot, I would think.

Brass was showing definate pressure signs but I kept thinking I was reading it wrong. This problem is no doubt exacerbated by the fact that I've shot an M14 for a long time. My 14's flatten primers on just about every load and put extractor marks in the cases. This is normal for a 14.

Furthermore, the necks really got beaten up. I need to take a closer look, but there appear to be bulges in the necks of some of the brass. I will take a caliper to them later. The primers also look cratered, but they are cratered all the way from the lighest load to the heaviest load.

I'm conjecturing here, but I can't imagine that having the shoulders whacked on chambering can be good for accuracy. As the chamber gets grungy, grunge is deposited in different amounts on the shoulder. Having a buildup of grunge on one part of the shoulder would cock the shoulder of the case and thereby knock the bullet off center in the cartridge. Let's say there's .001 grunge on one side and .003 on the other side of the shoulder of the chamber, the "dirtier" side would get knocked back more...theoretically.

So maybe I'll crunch some brass down to fit it. I use a small base sizer and it's nowhere near the amount of shoulder setback that it is capable of. Let's say that my chamber is 1.624. Where would you size your brass? 1.620? (it's an autoloader, remember)

By the way, the chamber was cleaned when I did the dummy round test.


Best group I got with it was 1.25 inch at 100 yards with LC brass, 41.5 grn of H4895, CCI-BR2 primer and 168 SMK's. Did that last week when I was getting the scope on paper.

 

[BCP]

Re: SASS with 1.624 headspace...go figure.

Well, I have the same rifle and new brass comes out a little shorter and fatter after firing. If it feeds, chambers and extracts factory ammo fine I don't think I'd worry.

 

[CST]

Re: SASS with 1.624 headspace...go figure.

I am not to convinced yet that you have a chamber problem. Even though DPMS will help fix your problem..your gonna loose your rifle for a long time till it gets back.

So...

First of all...if you havent already..you need to shoot factory match ammo like Federals match thru it and a chrono and see the groups sizes and chrono data and pressure signs as a reference...then work up the loads.(none of my guns like BHs and BHs shoot slow)

BHs dont shoot too well out of my Novekse .308 and only got me 2440fps with 175s. (I never shooot 168s)

All ARs are gonna have some pressure signs,,,you need to determine at which level is safe to use...I had primer flow on my Kreiger barrel into the firing pin hole on the bolt face....otherwise shot just fine at 90deg. A buddy had a Novekse 18" and kept blowing primers out with his 4895powder and LC brass...thats pressure!

My DPMS AP4 barrel shot my loads to 2430fps all the way out to 800 yards ...

My same load out of my GAP AR10 21"(rock barrel) and my 20"Noveske both shot at 2640fps with the same loads

Everyone of my ARs had some pressure signs....so what? Mild Pressure signs usually associated with bolt guns dont mean too much with ARs as long as it is ok at high temps and shoots consistently..you'll be fine. These arent M14s but they are just as finicky.

All the brass gets torn up really bad during extraction, necks are scarred up while being pulled out across the barrel extension and the body slamming into the deflector.

Hard to diagnos your problem without pics too.

With my 175SMKs I always neck size with a REdding bushing die to .331...If you have inconsistent neck tension you are gonna have inconsistent OALs too..all which will contribute to shots all over the place.

I dont like the ladder tests because it is too hard to shoot the AR consistently too..with all the human error...your better off trying factory match ammo first and work up from there then try the ladder or OCW or whatever test for load deverlopment you like.

168s should be able to be pushed over 2500fps no problems.

Maybe you should consider buying a field, go\nogo etc...gauge and check your chamber yourself.

Guys shoot AR .308s with 100s of rounds between cleanings and no problems...I usually clean every 75 rounds and my last group measure 3" at 500 yards...

 

[problemchild]

Re: SASS with 1.624 headspace...go figure.

My LMT is 1.614 if that helps. Measured 10 pieces of fired brass and that is the avg.

 

[aquinas]

Re: SASS with 1.624 headspace...go figure.

I still have the gun. I figured the turnaround at DPMS had to be slow. I just punched the primers out. Actually, the brass does not look all that bad. No bulges in the necks. I'm going to resize the brass to about 1.620 and use a pretty standard load. It's my m14 load: SMK 168's, 41.5grn H4895, CCI-BR2 primer. I'll shoot that again with the brass sized properly.

Thanks for the headsup on your gun chambered at 1.614. I feel better about that.

I don't know if you consider LC M80 ball "factory" ammo. It shoots about 2 inch groups with that stuff. No FTF's with the M80 and chambered just fine. I think the M80 goes in about 1.619.

Beautiful GSP. I own two myslef from a breeder in Nebraska. I used to go out to Iowa every weekend during bird season but lately been so obsessed with shooting, it eats all my time.

 

[problemchild]

Re: SASS with 1.624 headspace...go figure.

I think FGMM factory ammo is 1.608 new. You need to measure 10 pieces of once fired new ammo and avg the length at the shoulder.

 

[aquinas]

Re: SASS with 1.624 headspace...go figure.

And for those of you who don't know what OCW testing is(like me)...here's the link:

http://optimalchargeweight.embarqspace.com/#

GM308M 1.608 headspace??? Don't know about that, but it seems awful small. I once took my callipers and Hornady gauge to Gander Mountain and measured lots of boxes of Factory ammo for headspace and most of it ticked 1.624...somewhere in that area.

Just loaded that 50 rounds back up for another test, this time with headspace of about 1.620-1.619. Seems to feed fine. Gonna bump the ladder test back to 300 yards and see what I get. Thanksgiving week and all...don't know when I'll get to the range. Been raining too...so shooting takes longer since we can't drive to the targets. Gotta hump it. Spoiled, spoiled, spoiled. Let you know how it goes.

 

[problemchild]

Re: SASS with 1.624 headspace...go figure.

Just measured 2 new factory rounds. Round 1 was hornady match and round 2 was FGMM.

FGMM= 1.6045
H=1.6075

 

[BigSoundRacing]

Re: SASS with 1.624 headspace...go figure.

DMPS SASS I'm shooting 2410fps with SMK 175 backed by 4895 at 41.8, BR2, Winchester Brass - no signs of presure with or without taper crimp. Sub-MOA accuracy.

Full size each time with a Redding die.