Head Space and High Pressure

[gilream]

I have a group of 25 Lapua cases that have been fired 3 to 4 times in my old Remington 788 in .243 Winchester. In working up hunting loads over the course of those firings I have slowly approached, and exceeded a couple of times, the published powder charge limit. I've never experienced a loose pocket or other obvious signs of pressure. However, after depriming and (ultrasonic) cleaning the cases yesterday I was measuring the headspace on the fired cases and two of the cases were noticeably longer than the other 23. (I measure headspace with the Stoney Point (now Hornady) caliper attachment that comes caliber specific.) The two cases concern me. The brass is pretty consistent and all but two cases average 1.6235. The two cases in question measure 1.6255. They stand out within the lot as being noticeably longer at the shoulder dimension. I attribute this to high pressure in a past ladder test. I realize that I could be totally wrong with this assumption, it could just be the brass properties or difference in my annealing operation). I always use a Lee Collet Die (the case necks are not turned) followed by a Redding Body Die.

My question; After watching my shoulder bump very carefully and FL sizing all of the cases to a headspace dimension of 1.6225, would you expect these two cases to react differently on the target? Should I just toss them to take that doubt from my mind? What would you do?

I recently pillar bedded the old gun and it was like flipping a switch. She is a really fun shooter.

I look forward to your comments
Mike

 

[bugman]

Will those two sized cases chamber "correctly" in your rifle? By "correctly", I mean....if you take the firing pin assembly out of that bolt, and you chamber the sized cases only, will the bolt close and open as it should ?(closes with VERY light finger pressure, and no clicking sound at the top of the bolt-lift to extract the case. Bottom line is...are there any differences to the felt closing and opening of the bolt with those two cases compared to the other 23 cases you just sized?

My experience has been (with cases blown out that much differently), that you may be able to regain the correct amount of sized bump on those two (annealing sure helps), but the web of the case may be blown out too far for your die to get it back to where it needs to be. If that is the situation, you will get the "click" sound when you raise the bolt to extract the case.

Again, IMO,if your die will not size the over-stretched web correctly, then you should cull those cases .....or, if you want to TRY to save them, get another FL die or a small base die. YMMV, and as you said this is a hunting rifle and not competitive BR.

Jack

 

[fdshuster]

Mike: It's been my experience that as brass is used, and the number of loadings increase, you will get small differences in headspace lengths.

I've particularly noticed this in my Lapua 6ppc & 6BR brass that has 40+ loadings. I too am using the Stoney Point/Hornady gauge & always check headspace lengths before sizing, one reason being that I then know if I need to ad a die shim under the lock ring.

I've never noticed any differences in the results at the target, whether the brass is "High" time or "low" time.

 

[Steve Blair]

I have three Remington 788 rifles. There is an issue with high pressure loads in that action. The load path includes compressing the length of the bolt and stretching the receiver length. Contrast this with a front locking action that has a very short load path. As pressure increases, and particularly with Ø.473" case heads, the bolt will compress and the action walls will stretch. It's been years since I went through Otteson's book and calculated it but I seem to remember that 60 ksi produced ~.003-.004" total movement. As long as pressures are kept reasonable, brass behaves normally. When you try to push velocities in this action, the brass will stretch to the point that fired cases extract hard and will not rechamber. I do not own a .243 788 but have a .22-250, 6mm Remington and a .223. The .222 and .223 are great cartridges for the 788 action and have never seen a problem with them.

 

[jonbearman]

I had a nice 788 in 22-250 and it always stretched the case's after 3 firings they were failing with incipient head seperation.The headspace was correct and the action stretched enough to cause it with moderate loads.

 

[gilream]

I am aware of the history of this action stretching brass in high pressure loadings due to its rear locking lug design. (I wonder why they did that?) and I am not in the habit of going there. When I stated that I carried a powder test over the book max it was one grain over book and I got no primer pocket stretch. I have not and will not go there again. I'm pretty much a chicken when it comes to pressure. The overload was 43 grains of IMR 4350 with a Nosler 95 grain ballistic tip seated 15 thou off the lands.

The action is fairly heavy, small ejection port and the barrel has a contour to the heavy side

I have fired winchester brass 7 or 8 times and fortunately have never found a thinning case .

Thanks for the comments gentlemen. Lots of great information on this site.

 

[K22]

Your load (43.0 grains) is way over max published by Nosler (i.e. 41.5 grains of IMR 4350). Just to be safe, I'd discard the affected cases, there is no sense in taking any chances.

IMR 4350 is my go to powder for the 243 and I've never found it necessary to push the pressure envelope with this powder to acheive excellent accuarcy. My rifles do best with powder charges at the mid point between the max and min recommended for this powder.

 

[gilream]

K22...I have a Nosler Reloading Manual number 3. It was printed before the ballistic tip bullet was introduced so I used the data for the 95 grain partition bullet and the 100 grain solid base (exactly the same). It states the maximum charge for IMR 4350 and the 100 grain partition bullet is 42 grains. I am shooting the 95 grain Ballistic Tip.

Did the data change that much with the advent of Ballistip Tip bullets?

 

[gilream]

K22...I just looked online at Nosler's reloading data. The online data does not list IMR 4350 for the 95gr. Ballistic Tip bullet however it does list a change from the older manual with its IMR 4831 data. IMR 4831 in the older manual Nosler lists the max loading at 44 grains for and the modern online version drops it to 42 grains. Has Dupont (or whoever owns IMR powders now) changed the burn properties of these two powders over the last 20 years? What gives? Lawyers?

BTW..I throw all powder charges first with an RCBS Chargemaster digital and weigh a second time on an Aculab Vicon-123 before dumping it in the case. Overboard I suppose but I feel pretty confident in my charges.