I posted this a few days ago in the gunwriters section but didn't get much traction. I wasn't sure what the best place to post this, gunsmithing, hunting rifles, or where. I landed here for now.

My FIL handed down his Zvasta 7mm rem mag that he had built in the 80s to my 10 year old son. He had several boxes of factory ammo as well as some reloads. We ran a few rounds of the reloads through it as all the factory rounds were 175gr and these were 150s I think. I wanted to shoot the lighter ones because my son wanted to shoot it so bad he couldn't stand it and figured they would be slightly more tolerable in the recoil department.

On the 3rd or 4th round the case let go. Gas came out the floorplate and some smoke rolled out of the action. The bolt came back with the head of the case. I just got the case out using a 45acp brush. After that happened I remembered my FIL telling me that he had a headspace problem but the gunsmith told him to just use factory ammo and not reloads.

So my question is: If that is the case wouldn't the problem go away if you didn't full length size your brass all the way down and sized the brass to fit the chamber?

With a belted case how can that be a problem if it headspaces on the belt if the shoulder isn't in contact?

I was wondering if it was just brass had been reloaded too many times and was a case of head seperation?

I did shoot some factory 150grain corelockts through it. I was wondering about using a 45acp or 40S&W case to attempt to measure the once fired dimension to a point on the shoulder then measuring the resized case to see how much a shoulder is being bumped back?

I would like to work up some lighter 120 grain loads for the lad but don't want to have another blow out. I think the last one may have bowed the floorplate slightly but didn't look till after it happened. It could have been that way from when my father in law had it.

Technically the belt is the head space part of the case but when you light up the shell the pressure blows the case forward to the full length of the chamber. It usually stretches from just ahead of the belt and this thins the case wall. Over sizing can set up the scenario all over again.
I highly recommend spending $35 on a hornady head space measuring kit.
You will quickly see how far forward your cases are expanding...then you can set your resize die for minimal shoulder bump to stop future stretching.
Personally it should almost be illegal for anyone to load belted mags without a head space measuring tool...they are that handy.

Probably the reloaded rounds were FL sized and most know when you haev a belted case, after the intial firing ,you do not FL resize,but partialy resize or neck size.

The 7mmRM is said to headspace on the belt. That is for sure on the first shot, because the shoulder on a new case will not likely reach the shoulder in the chamber, before the belt on the case can reach the belt in the chamber.

That is not to say that the case belt and the chamber belt are close, they are not. SAAMI specs are sloppy


.220 - .008" for cartridge belt
.220 + .007" for chamber belt clearance
---------------------
Belt specified to have between .000" and .015" clearance

2.2700 - .0070" for cartridge shoulder clearance
2.2791 + .0100" for chamber shoulder clearance
---------------------
Shoulder specified to have between .0091" and .0261" clearance


The brass I measure of many samples of many brands looks like .210" to .215".
So when I cut threads and the cut chamber on a 7mmRemMag rifle, I headspace the rifle at .215".

But the factory rifles are all more than .220"
I have a Ruger #1 7mmRemMag.

So what to do with a factory rifle?
The first time brass is fired, the belt will stop the forward movement of the cartridge when the firing pin pushes.

Partial neck size, neck size, or FL size and push the shoulder back .001".
Then on the next firing, the shoulder will be what stops the cartridge forward movement in reaction to the firing pin.

What does it all mean?
If you touch the die to the shell holder, after a few shots, the case may break in half.
You have to screw the sizer die a little higher in the press.

I have a Wby that has the same issues....just ordered a neck sizing die from Huntington Dies. FLS to just bump the shoulder, still moves way to much brass from the fired and extended case to do much good in preserving case life for too long.
I think with the belted Mags neck sizing only may be the way to go. And you can still bump the shoulder a bit.

I had a Winchester model 70 westerner that had a long chamber. I knew it and did a partial FL sizing to get it to fit without bolt closing resistance. First shot fireforms it to the longer chamber. It was so long that it wouldn't allow my friend's Mark X to even begin to close the bolt! Cases lasted 5 or 6 firings.

If it really bothers you buy a custom reamer to your specs, setback and rechamber.

It`s not a problem after one understands what`s happening. I`ve some Wby headstamped cases I`d like to preserve as long as possable..I see this as a solution.

Thanks

It's a little hassle, but the necks of new cases can be expanded using a tapered mandrel. Then using the regular die they can be sized back just enough to allow the bolt to close with a little resistance.

Thanks,
I have fired a half dozen factory ammo. I don't have the hornady headspace gauge and it looks like a good tool to use. I will look into getting one. For now I will just use an empty pistol brass to get a reference dimension and resize till I get a minor set back with my full length die. A partial full length size I guess is probably the term. Check the bolt for slight resistance.
That is how I do my other cartridges but with blowing brass right away and not having reloaded yet for this rifle I was nervous about there being issues I wasn't aware of. Looks like this will take care of it.

I will let you know how it goes and am going to scrap any reloads that were given to us. That part makes me too nervous.

This is a prime example of why you should never shoot any reloads except your own unless you truly trust the individual that loaded them. Any mention of a headspace issue should have raised a red flag.

I wouldn't get the neck sizer, I'd stay with the FL sizer and partial resize. I don't know that going to a neck sizer that would size all cases right would solve the problem. I'd shoot a factory case and then size about 3/4 way down the neck to the shoulder. Make sure the bolt close's easily on the case. Fire again and do the same again. At this point the bolt will probably close with with a bit of rubbing, probably will see some bras on the bolt face from the rubbing. Set the fl die down about 1/8th to 1/4 turn at a time and run that case thru until the rubbing just quit's. At that time the case will be head spacing in the shoulder. Forward movement pretty much quits right there. Been a problem with belted cases since they started making them. Good idea to do it with rimmed cases in a bolt action also. Had a Hornet years ago that was death on case's until I started partical sizing. I don't size rimless case's like that unless head space rear's it's ugly head.

I have many lee collect dies and use them in combo with redding body dies ....its a great way to go. You can control what you size and when...all with cases prepped to virtual zero runnout!

x2

Best way to go