7.5 x 55 Swiss Case Separation

[PCCUSNRET]

Hodgdon data does not list Varget. Varget would seem to fast, and your load to hot. Different brands of brass can give high pressure with starting loads.(Prvi Partizan in 308win) H4350-44gr. w/ 168gr looks like a better/safe load. I do not own or have i ever owned a 7.5x55, so take it for what its worth.

Varget is definitely not a "hot" load in this rifle. The rounds I shot yesterday with "Varget" were not the ones showing case head separation, they were new GP-11 rounds. I used Hodgdon's .308 Winchester load data for developing loads for the Swiss 7.5 x 55 along with recommendations from many that have used this powder with good results. This round in a K-31 is very accurate and I really don't believe it is causing the case head separation (but I've been wrong before). Thanks for your input.

Best,

Chuck

 

[PCCUSNRET]

So that case you sectioned is only once-fired? In your initial photo, you mentioned the round on the left had signs of separation, but I don't see it. The empty case next to it does, but not the case on the left. All fired/sized cases will show a pressure ring in that area. The trick is controlling it.

The cases in post #15 appear normal, btw.

The case I sectioned had only been reloaded once. You are correct, the round on the left didn't show signs of separating, it was the case just to right of the loaded round that shows signs of separation. I understand there will be signs of pressure in this area, but should there be that fine white line? I sectioned this case also and though it wasn't as bad as the one in the first posting, there was a defined "groove" in the brass where the line showed from the outside of the case. I personally believe this case would probably separate if I reload them but I may give it a try to see if I'm correct. I'm just hoping it isn't a serious problem with the gun. I really enjoy shooting this gun and if I do my part it is a tack driver out to 100yds (probably further but I can't see much further than a 100).

 

[PCCUSNRET]

How about the twisted cartridge, anyone ever see anything like that? There are 8 dents in the case and they appear to spiral around the case.

 

[NCsmitty]

The dents in the case in the picture appear to be from pressure bleed past the case neck leaving a soot trace. The case appears not to be obturated enough to seal the chamber, and that usually happens when the load is not developing enough pressure. Hard to believe with 45gr of Varget.

Something irregular is occurring with this thread, and it shouldn't be this difficult. Originally it seemed that it was a simple matter of the OP creating false headspace from setting the shoulder back and subsequent case separation. Now we hear that new GP-11 ammo is nearing case separation when fired. HMMM....
Also, you should give pause when you do not find loads listed on the Hodgdon site, for using Varget in the 7.5x55. HMMMM......
Perhaps a cerro-safe casting of the rifle chamber(s) should be done to see where and what the chamber dimensions look like.


NCsmitty

 

[918v]

The groove in the sectioned case looks bad after just two firings.

Depending on the headspace setting of your rifle and the tolerances of the ammo, you may have a situation where the rifle and the ammo are at the maximum end of the spec. Also, combat rifle chambers are on the loose end for reliability's sake.

I believe all of your once fired cases will stretch that way. If you bump the shoulder of your reloads by .003" or less, the cases will remain intact.

Also, you may be experiencing the teeter totter effect of too much shoulder bump/ expander ball pulling the shoulder forward. You really need to lube the inside of the necks, otherwise the whole geometry of the shoulder will change and the case will stretch more.

 

[mikana]

Here's another thought. I have a cheap lee die that I use to reload for my K31. It worked ok, but was a little finicky so I sent it in to Lee with two fired cases. I got back a retooled die that is cut for my chamber within about 7 days. Might be worth a thought. Don't know if other manufacturers have a similar guarantee.

 

[243winxb]

I used Hodgdon's ".308 Winchester" load data for developing loads for the "Swiss 7.5 x 55"

Ok, that explains where the Varget came from.

 

[R.W.Dale]

Ok, that explains where the Varget came from.

and with due caution there'd be nothing wrong with the practice as k31's have been chambered for higher pressure rounds and 7.5x55 has more case capacity than 308win. But on the low end of the data it could be an explanation for the low pressure case collapsing.

 

[mikana]

Not that it means much, but my sierra manual doesn't list varget either. FWIW

 

[918v]

His load is generating about 40,000 PSI per QuickLoad.

 

[243winxb]

Would using another bolt change the head space?

Yes, not a good idea as each bolt is matched to that firearm. Post #5 >

I also have a neck sizer for this round but it normally only works once and then I have to full length resize.

I find this odd, maybe to much pressure? Or is this common with a straight pull rifle action? After rereading everything Your rifle has excessive headspace because factory ammo is showing signs of case head separation. OR you have bad ammo, try a different make/brand/lot of factory ammo. When fire forming for a wildcat, some put a light oil on the brass. This keeps the front of the case from grabbing the chamber wall and the brass can not stretch causing case separation. If you do this then adjust your FLRS die to Partial Full Length Resize, it might stop the casehead separations.

 

[R.W.Dale]

It's common with a straight pull rifle to only be able to FL resize as they are pretty much a manually operated semiauto

 

[Slamfire]

I have not found anyone making cartridge headspace gages for the 7.5 Swiss. I tried to figure out the "headspace" on two K31's that had mismatched bolts and receivers.

In doing so, with my Stoney point gage, I found that new Norma, Indep and Swiss cases get stretched one heck of a lot on first firing.

Too bad the formatting is lost when I copy this, but I am not surprised that cases are seperating when cases are stretched as much as .014"

7.5 X 55 Swiss Cartridge Headspace Measurements		
					
Using "C 375" Stoney point gage	add 2.001" for gage length		
					
Unfired cases	          Swiss "T"	Norma 	Indep		
					
	                        1.8185	1.823	1.828		
	                          1.82	1.8235	1.828		
 	                          1.8205	1.824	1.828		
	                          1.821	1.824	1.8285		
	                          1.821	1.8245	1.8299		
	                          1.8215	1.8264	1.8295		
	                          1.822				
	                          1.822				
					
Measurement Average	1.820	1.823	1.828		
					
Difference between fired 	0.014	0.011	0.006		
and unfired in rifle # 622XXX					
a rifle with matching numbers				
					
					
K31 Rifle	#622 XXX	#718 XXX	#869 XXX	#869 XXX	
					
	Indep Brass	Swiss "T" Brass	Swiss "T" Brass	Swiss "T" Brass	
	1.834	1.832	1.8305	1.8315	
	1.835	1.833	1.8305	1.832	
	1.8355	1.8335	1.831	1.8325	
	1.8355	1.8315	1.8305	1.8325	
	1.837	1.8315	1.831	1.83	
	1.8375	1.832	1.832	1.8305	
		1.832	1.832	1.831	
		1.8315	1.831	1.831	
		1.832	1.8305	1.833	
		1.8315			
					
					
Average Fired Headspace	1.834	1.832		1.832	
					
					
Difference between fired 	0.008	0.011		0.011	
and unfired cases

 

[counterclockwise]

This sounds like the .303 enfield reloaders dilemma.

So what do you do? Perhaps:
1. buy new brass, neck size, trim, load, and fire;
2. neck size only with trim on 1X, 2X, etc. until bolt won't close;
3. bump shoulder back slightly, load, and fire, go back to #2.

Create as many separate product streams as necessary to address odd-size chambers you are dealing with.

 

[PCCUSNRET]

I want to thank everybody that provided suggestions and advice in regards to this problem. I do want to clarify one thing, the bolt in the rifle used to shoot these rounds does have matching serial numbers. The reason I asked about using a different bolt is that I have several lying around that I got from "parts" guns that I was considering using if it would help to improve what appears to be a head space problem in this gun. I guess my question is now, is there anyway to correct the head space problem without replacing the barrel? Thanks again, you'all have been very helpful!

 

[918v]

Nope.

The camming surfaces are machined into the barrel extension which, unfortunately, is an integral part of the barrel. You can have a custom barrel made by chopping off your existing barrel near the shoulder, boring and threading the barrel extension, and installing a new barrel in the old barrel extension.

But I don't think your cases will fail if you size them correctly after the initial firing. I know that groove looks scary, but you can keep it in check by bumping the shoulder minimally and neck sizing with a Lee collet neck die inbetween shoulder bumping.

 

[Slamfire]

The reason I asked about using a different bolt is that I have several lying around that I got from "parts" guns that I was considering using if it would help to improve what appears to be a head space problem in this gun. I guess my question is now, is there anyway to correct the head space problem without replacing the barrel? Thanks again, you'all have been very helpful!

On Swiss Rifles I asked about headspace gages for K31's. Two of my K31's have mismatched bolts. Turns out headspace gages are rather rare, because no one needs them. Posters stated that all bolts for K31's will meet headspace in any rifle.

Then I conducted my test. The mismatched rifles had tighter chambers than the matching.

This is just beyond belief that in a production lifetime of 600,000 rifles and 20 years that bolts are all interchangeable. But, it could very well be.

Built like a Swiss watch

 

[PCCUSNRET]

So what do you do? Perhaps:
1. buy new brass, neck size, trim, load, and fire;
2. neck size only with trim on 1X, 2X, etc. until bolt won't close;
3. bump shoulder back slightly, load, and fire, go back to #2.

Here is what I'm going to try next:

1. I am going to try some resized Win .284, Privi Partizan and Graf brass in this gun to see if I get the same results.

2. Tried neck sizing with these guns and have found that for most of them I only get one neck sizing and then I must do a full length resizing.

3. I will probably try this instead of neck sizing if I can find a case that works in this particular gun. I was going to try this on the brass from the rounds that I just fired but I've decided these will go to the scrap bin since I believe they will separate once reloaded and fired in this gun.

 

[PCCUSNRET]

On Swiss Rifles I asked about headspace gages for K31's. Two of my K31's have mismatched bolts. Turns out headspace gages are rather rare, because no one needs them. Posters stated that all bolts for K31's will meet headspace in any rifle.

Then I conducted my test. The mismatched rifles had tighter chambers than the matching.

This is just beyond belief that in a production lifetime of 600,000 rifles and 20 years that bolts are all interchangeable. But, it could very well be.

Built like a Swiss watch

You would think this would have to be true with the number of mismatched bolts that were sold in the States. I'm not sure they were shipped over here that way but I know I have personally received 3 with mismatched bolts. I have yet to fire any of these but surely someone must have decided they were safe to sell as they knew we would be shooting these guns. I took advantage of some of the sales they had for "cracked stocks" and "parts" guns and all of these guns either had mismatched bolts or looked like they were left setting on the pier (some still had water in the barrels). I may just try changing the bolts to see if this helps reduce the stretching of these cases.

 

[PCCUSNRET]

Nope.

The camming surfaces are machined into the barrel extension which, unfortunately, is an integral part of the barrel. You can have a custom barrel made by chopping off your existing barrel near the shoulder, boring and threading the barrel extension, and installing a new barrel in the old barrel extension.

But I don't think your cases will fail if you size them correctly after the initial firing. I know that groove looks scary, but you can keep it in check by bumping the shoulder minimally and neck sizing with a Lee collet neck die inbetween shoulder bumping.

This is what is suspected but wasn't sure. I may try a few of these "once fired" cases with a slight "bump" and see what happens. I will also try the ones that were fired in another rifle the first time and see how many times I can reload them for this rifle. I was really surprised to see that this brass doesn't show the same signs of head separation as the newly fired brass. I suspect this would be caused by the initial stretching of these cases wasn't as great in the gun they were fired in the first time (I bought this brass from someone online). This brass was trimmed and then resized with Redding dies.

I've gotten a lot of good suggestions and will now try a few different things to see if I can sort this out. Thanks again for all the help.

Best,

Chuck

 

[PCCUSNRET]

Just a quick follow up on what I've discovered since my initial post about this problem. I tried some Prvi Partizan rounds this past weekend in this gun and found no signs of any case head separation. The cases (GP11 and Prvi) seem to have stretched the same distance but the Prvi brass didn't show signs of separation like the GP-11 brass. I tried some of the GP-11 from the same lot in a couple other K-31's and found no signs of separation. So if I want to keep shooting this gun I see my choices of either getting a set of dies made for this rifle or using only Prvi brass with minimum bump of the shoulder when it becomes difficult to chamber from neck sizing? Notice the difference in shoulders from original GP-11 round, Prvi Paritzan and fired cases? Thanks again for your indulgence and help.

Chuck