Hi All,
I am working up loads for my .378 weatherby and I have a question regarding case expansion. Is there a maximum allowable for case expansion, I am measuring with calipers just ahead of the belt. Is this a non issue or a possible way of tracking max. pressure.

Not a good way to track max pressures. Stick to a good reloading book and look for pressure signs on the primer , bolt extraction, check a chrono.Go for a ggod accurate load rather than MAX.

The major issue I've found with "head expansion" is that the pressure at which it happens changes from one brass manufacturer to another and often between lots. When I was using pressure equipment I would see expansion start sometimes in the low 60s and sometimes as high as 72-75,000. My caliper is not near accurate enough to measure .0004". I use a micrometer.

For my wildcats I always use 280 Norma brass. I bought 100s of the same lot years ago. I know from testing what expansion for that batch happens around 65-67,000 so that is my limit.

On other cases since I have no point of reference I watch for other signs and lean very heavy on the manuals.

I know many say as long as you have no expansion the load is safe. I simply choose not to load to 70,000+ period.

I doubt measuring in front of the belt with a caliper will give you much useful information.

quote:

Originally posted by BaDonkaDonk:
Is there a maximum allowable for case expansion.

Generally speaking, .0005" is max using new, never fired brass.

quote:

I am measuring with calipers just ahead of the belt. Is this a non issue or a possible way of tracking max. pressure.

Someone will correct me if I'm wrong but, I think you should be measuring the diameter of the belt, not forward of the belt. Where you are measuring, the brass is quite a bit thinner than the belt which is sold but for the primer pocket and flash hole. If you exceed .001", I think you are over pressure. We (my shooting friends and I) would discard any brass that exceeded .001" at the belt and reduce the load accordingly.

A caliper is inadequate for resolving fractions of thousandths of an inch. In four books and five descriptions on the web about measuring case head or pressure ring expansion that I have seen there are six different specifications for measurement thresholds. Stick with the advice of the first response.

How many of you guys have ever measured the belt expansion of a factory.460WBY? Your in for a RUDE awakening! The belt in a factory cartridge will expand often to .003-.005! That is despite this being the strongest and thickest part of the case head. I've had 7mmSTW's go .002-.003 in brand new Rem 700's. How much depends on the chamber ( how it was cut) and the hardness/thickness of the brass in those cases. Interestingly, the primer pocket stays tight usually for four reloads after that, then loosens fast! Factory chambers are the worst and custom chambers usually better. The problem is exacerbated by the uniformity of the brass. I've found the best approach to develop a belted magnum load is to fire 10 factory loads and take a average of the BELT expansion. Each gun will be different! Thats now Max expansion with a new case.! It will surprise you. Now develop your reloads using fresh brass of the same type and limiting load development till you reach the same belt expansion. I set my loads so that I can get 5 reloads from the same case without any loose primer pockets or sticky bolt lift. Interestingly, subsequent firings of the same load may show another .001 of belt expansion then stop. Of course the brass is now work hardened and is resisting the expansion more and more. If you keep reloading the same case you'll eventually find the primer pocket gets real loose and after that you'll get head separations about .250 above the belt. Guess where all that brass is going? Up and out! My rule is that After 5 firings , dump the cases!-Rob

Thanks for the info everyone, very interesting.
Rob, I just measured my belts and all of them have expanded .004 - .005" compared to new brass. Based on your info that must be what my chamber has been machined to. My primer does not show any pressure signs what so ever.
Here is some info from the chrono yesterday evening:
260gr Accubond
215 Fed. primers
RE22 powder

115gr 3328,3321,3321 fps
116gr 3319, 3339, 3368 fps
117gr 3427 last 2 rounds didn't measure possibly due to low light.

I am new to big magnums and am very impressed with this cartidge. The new 378 with the heavy barrel and muzzle brake was like shooting a 338 mag. Very manageable. Pulled off the muzzle brake with 117gr and I knew I had a tiger by the tail though. Now that I have experienced the recoil time to work on accuracy.
Thanks again for the info
Rog

Yup,just as predicted. Just follow my advice and you will have no problems. Remember a safe load is acheived when you can reload a case 5 times and the primer pocket is still tight. If it loosens up back off pronto. If you ever have sticky bolt lift, back off. Extensively flattened primers are not a good sign either in a belted magnum despite what some might say. Trust me, I've pushed the envelope many times and know whaT I'M TALKING ABOUT. You need to be able to read all the signs when you develop loads for these guns. THOSE 378 WBY'S DO KICK A MITE DON'T THEY? -Rob

Sounds like Rob as it covered, just want to skip in that when operating a bit under max, cases will last a lot longer. One of my friends .378's have cases reloaded as many as 20 times.

You need a copy of the Hornady reloading manual Third Edition,this has a erudite explanation of the technique of case head expansion measurement.Then get a proper BLADE micrometer that measures down to one TENTHOUSANTH of an inch and combine that with a chronograph.You will find,as I have with my 378,that to match factory ballistics requires loading to the MAX.I would suggest aiming about 100fps below.I have had some older factory ammo. melt the case head into the ejector button in hot weather (40 deg C) here in Australia.
The comments about variations due to brass lots are entirely accurate.

Thanks guys,
Rob, I am going to load 117gr in the same brass from the previous 117gr firing and check for additional case expansion, I think I will probably make that part of my load work up procedure.
Hey Rob & Wombat, Just out of curiosity what loads have you worked up to and with what bullets? The reason I ask is this caliber appears to be quite multifunctional as far as handling smaller & larger gr bullets.
Roger

Measureing case expansion is the best way to know where your pressure is for your particular rifle when working up max loads...

.0005 is the accepted standard although I know many folks call .0007 the max..

Alway use new brass..

Useing this method with a chronograph is the best way to go IMO..

Most important is use a micrometer not calipers.

Approach the .0005 with care, looking for the other accepted signs of pressure, don't lock yourself into a single method of recognizing pressure, use your head and you will be able to customize your loads.