Funny pressures with IMR 4350?

[redneckclimbing]

So, my brother-in-law and I were working up a new load for his .243 Win. Using Federal brass (match prepped), CCI BR-2 primers, IMR 4350, and 90gr. Scenars. Everything was from the same lots.

I didn't have exact load data for Scenars so we were more or less using the data for 90gr. flat base spitzers from the #10 Speer Manual.

We started at 42.2 grains of powder and worked up in .3 grain increments. (The min. load in the book is 41 and max 45).

After shooting the first four bullets, (two each at 42.2 and 42.5), I thought this isn't going to be a very fast barrell. All four primers were completely flattened! I had never seen primers that flattened before! So, I told my brother-in-law that we would have to be really careful as we go up from here.

Well, as soon as we got to 44.1 grains all our problems stopped! Primers looked completely normal, not hard to lift the bolt at all, NOTHING?!

We worked loads all the way up to 45.6 grains before the accuracy started dropping off again. And the primers with 45.6 grains in them looked great compared to the 42.2 grain primers. (They were just starting to get flattened a little, but still OK.)

Anyone ever had this happen to them? Any explanations?

By the way the gun was a 700 CDL 1:9.25 barrel. The temp. was about 90F (but I thought that 4350 was supposed to be fairly temp. stable?).

 

[redneckclimbing]

By the way, we were getting sub .5 MOA accuracy with the flattened primers aroiund 43 grains. And then also again around 45.3 grains.

Not too bad for a bone-stock gun, a makeshift load, and me behind the trigger!

 

[Lefty7mmstw]

Often primer flattening has more to do with headspace than pressure. If you have rather low pressure and a few thou. headspace, the primer has time to back out before pressure pushes the heel of the case to the breech face. Doing so, it flattens the primer. If your load starts out with rounded primers AND they progress to flat, the pressure is going up. If they start out flat with sub-par speed and get better(or stay the same) as the speed and charge/pressure go up, you have a bit of headspace in your set-up. You can either set your die out a bit or leave it depending upon if your case growth is excessive or not.

 

[redneckclimbing]

That makes sense as they did seem to protrude just slightly more then the others. I will see if I can confirm this with a headspace gauge and then adjust my die accordingly.

Thanks a bunch!

 

[Joe King]

Funny thing about powder charges and pressures is that if your powder charge is to light it can cause huge pressure spikes, that occur later in the combustion and do some really odd things. Did you notice any resistance to bolt lift with the lighter charges?

 

[Mikecr]

You're seeing first hand that primers alone are not very good predictors of pressure.
I liked that you continued on, given flattening primers as a single indicator that did not represent a pressure 'problem' in itself.
Good observation

 

[redneckclimbing]

Joe-

I would say that it was stiffer at the lower charges than in the middle. However, they were really close, and then again at the top end you could just start to tell again.

I really appreciate all the info., still have not been able to check the headspace exactly but I will.