m305 primers pushed out a bit with .308 factory ammo

[Winston1984]

Hi all,

I keep good track of this thread and appreciate your insights. Just looking to expand my knowledge a bit here.

Equipment is a stock m305, 2012 vintage, with cases measured at "Your brass sizes : 18, 17, 17 thou" (Thanks Tactical Teacher).

Had it out and accuracy-wise it did pretty well. I was looking at cases and picked a few up.





On the left are copper norinco 7.62 NATO. They look fine. On the right is MFS steel cased .308. With the MFS rounds, the primers are 'pushed out.' Everything else looked normal, no cracks, etc.

I saw a neat animation in another post that showed the base during firing. It looked like it pressed back against the face of the bolt. So I was surprised to see this.

Any ideas of why this is happening? Headspace? Ammo differences?

Thanks for your input.

WS

 

[yomomma]

Its probably the ammo...flat primers is what you have to watch out for.

Mind you 18,17,17 is on the large size

 

[sunray]

That'd be the ammo. Generally, if one brand gives you grief, but not another, it's the brand. That MFS stuff is over pressured if the primers are backing out. Or are not correctly sized.

 

[flashman]

That MFS stuff is over pressured if the primers are backing out.

Sigh.

No, it indicates exactly the opposite. Backed out primers are an indication of LOW pressure and long headspace. If it was excessive headspace and HIGH pressure the primers would be flattened.

OP, your chamber has long headspace for .308 based on those measurements. The ammo seems to be low pressure; the case is gripping the walls of the chamber and the steel isn't stretching enough to meet the bolt face - so you have the primer backing out until it meets it. No biggie, just watch a primer doesn't come all the way out and gum things up.

 

[Winston1984]

Sigh.

No, it indicates exactly the opposite. Backed out primers are an indication of LOW pressure and long headspace. If it was excessive headspace and HIGH pressure the primers would be flattened.

OP, your chamber has long headspace for .308 based on those measurements. The ammo seems to be low pressure; the case is gripping the walls of the chamber and the steel isn't stretching enough to meet the bolt face - so you have the primer backing out until it meets it. No biggie, just watch a primer doesn't come all the way out and gum things up.

Ok this is what I figured. I gather that a primer coming all the way out could be painful? How much tolerance is there? Is this really unsafe, vs. potentially unsafe?

Another question, what do you think brass .308 would do? Seems like a pretty long stretch. I had previously fired a few rounds of winchester white box .308 - noticably more snap to those vs. the MFS. Did not look at the cases though.

It is too bad that 7.62NATO is hard to find. Lots of bulk .308 around.

Thanks!!
WS

 

[yomomma]

Stay away from federal, thats all I can say. they are soft like a limp ####.

The two rounds above were fired from a +.014 chamber. I lapped in a new bolt and now my chamber is 1.360 on the nose(or +0). I bought about 2000 rounds of S&B which is great ammo for the price and re-loadable.

 

[flashman]

Ok this is what I figured. I gather that a primer coming all the way out could be painful? How much tolerance is there? Is this really unsafe, vs. potentially unsafe?

Another question, what do you think brass .308 would do? Seems like a pretty long stretch. I had previously fired a few rounds of winchester white box .308 - noticably more snap to those vs. the MFS. Did not look at the cases though.

It is too bad that 7.62NATO is hard to find. Lots of bulk .308 around.

Thanks!!
WS

Like YM says, I'd stay away from Federal, and add American Eagle in there. What you have is a very loose .308 chamber, the 18/17/17 thou over means that much over SAAMI max chamber length. That's a lot of case stretch. That's like machine gun chamber loose. The worry isn't a primer backing all the way out (primer height is about .118", you're backing them out .018"), rather it's case separation. I would not reload for this rifle, or reload any brass that had been through it, that's asking a lot of the brass.

My experience with the MFS is that it's loaded to normal M80 ball spec, 147grs at 2750 - 2800 fps. If you notice a difference between it and Win white box .308 that suggests this batch may be a little slow - the Win is be the same spec, 147gr / 2800fps. Does the MFS print lower? Can you chrono it?

Brass loaded to normal pressures will stretch no problem, again, that's just asking a lot of it. And possibly too much of some thinner .308 civilian brass, especially AE. You could expect to see flat primers as per YM's post, but without the other traditional pressure signs of ejector pin marks and primer cratering. Worst case, case separation.

If Barney didn't suggest it's a problem to shoot with that headspace, I'd say go ahead and enjoy, he's the M14 subject matter expert. I'd stay with the Norc ammo though - the MFS not stretching like that is risking case separation, as the case is taking the full pressure of the load without support from the bolt face. Could just be that lot though, it's impossible to know without a chrono or guesstimate via POI and reverse engineering through a ballistic program vs known full velocity loads. Never heard of MFS having issues in M305s, put a gun to my head and I'm leaning towards a variance in this lot?