Not your usual flattened primer question

[delta8221]

Hey guys, I've been reloading for a long time but this is my first time loading for precision, Gun is a 700 5R .300 win mag with the only modification being a muzzle break. I'm getting flattened primers every time I shoot, normally a sign of over pressure but it happens with calm loads and factory ammo of all types as well(hornady, remington, federal). I've been reading this could be a issue with too much headspacing and the solution is to tune the brass to the chamber. So I took my once fired brass and dropped them into a rcbs precision mic and the reading came out to 2.2791 ansi minimum. Wouldn't this mean my chamber is tighter than normal? If my chamber is tighter than normal what am i missing that's causing my flattened primers? Load is a 190 smk with several powder charges h1000 between 76.6 and 81.2(been doing some load development)

Thanks in advance.

 

[BuzzBoss915]

Re: Not your usual flattened primer question

Total mystery, if you have reduced the load and they still flatten? Seems like I have heard of this but can't recall the exact specifics? It does not seem normal? For me, flat primers and high pressure go hand in hand but we are told that some gas guns cause flat primers without excessive pressure? BB

 

[RotARy15]

Re: Not your usual flattened primer question

I pay attention to flat primers when they show up at hot loads. If they are appearing at low pressures, I dismiss them altogether and look for ejector marks.

 

[aloreman]

Re: Not your usual flattened primer question

measure for case head expansion

 

[Hookturnr]

Re: Not your usual flattened primer question

If you have a VERY tight chamber you can find you will hit max pressure well below book max with a few different powders. I've seen this with one of my rifles using both Retumbo and H1000. These powders seems to go to critical pressure extremely quickly at times. The chamber in my RUM mics at exact min. saami spec. I get max. pressure signs 2.5 grains below book max with one powder and 3 gr. below with another. Shoot it over a chrony and check for ejector marks on the case head. Don't be afraid to load a little lighter for the sake of consistency and accuracy. One of 300RUM's will push a 208 amax at 3200 at 1moa but back it down to 3060 and it drops to .5 moa or better! Good luck!

 

[turbo54]

Re: Not your usual flattened primer question

If I'm reading the OP correctly, he fired cases, had flat primers, then measured the fired cases to find they mic'd right around SAAMI minimum....?

If that is true, what was the case headspace length prior to firing? I have indeed found that a case that headspaces loosely will tend to flatten the primers. Perhaps they fit loosely, but have now stretched to fit the chamber? If that is true, then perhaps the fireformed cartridges won't flatten the primers...

That all said, I cut my reloading teeth on 308 and found that flat primers were a pretty decent way to judge pressure. Particularly once I had a feel for the rifle and what it was doing, I could definitely see a difference in primer flatness based on charge weight. In 223, it seemed to work the same way.

However, as I've stretch my wings out and began loading for other cartridges, I've been seeing that flat primers don't really apply to all cartridges. Perhaps it's rifle dependent, perhaps it has to do with powder burn speed, I don't know...but I can tell you this:

Regardless of "case headspace", flat primers are ZERO indication of pressure in my 284. Primer cratering is the ticket in that rifle.

Ejector marks come long before flat primers in my TRG42 338LM with 300smks and Retumbo.

I quit going up "the ladder" with my 260 at 44.2gr H4350 under Hornady 140hpbt with no primer flattening, no ejector marks, and no primer cratering. I quit because I know "what's reasonable" in 260 based on the thousands of folks here on the 'hide that shoot it.