Datum line for 7.5 Swiss ?

[Canuck-IL]

If anyone has a cartridge / chamber reference source, I'd really appreciate knowing the datum line, ie. the width of the cartridge on the neck from which the headspace is calculated, for the 7.5 * 55 Swiss round.

Trying to figure out if one of the Hornady headspace gauge bushings will work w that cartridge.

TIA
/Bryan

 

[Slamfire]

I guess Hornady has purchased the rights to the Sinclair Headspace gage.

You are running into the same problem I had, I could not find Wilson type cartridge headspace gages for the 7.5 Swiss, so I had to figure which Sinclair collar to use.

I had two K31’s that the bolt and receiver numbers did not match. So I was concerned about excessive headspace, for one thing.

It has been awhile, but I picked the collar that fitted midpoint on the shoulder. And I don't remember which.

I compared the length of unfired swiss, norma, Indep cases to fired Indep cases in a rifle that had matching bolt and serial numbers.

What totally knocked my socks off was, the Swiss built these rifle to a level of precision that I have never encountered before. After measuring case expansion with a “known” rifle, against the rifles with unmatched bolts, the unmatched rifles had tighter headspaces. I had been told that K31 bolts are totally interchangeable over the 25 + years, and 800,000 or so rifles, and it could very well be. Incredible.

7.5 X 55 Swiss Cartridge Headspace Measurements		
					
Using "C 375" Stoney point gage	add 2.001" for gage length		
				              K-31 #622 XXX	
Unfired cases	Swiss "T"	Norma 	Indep	Indep Fired	
					
	             1.8185	1.823	1.828	1.834	
	             1.82	1.8235	1.828	1.835	
 	             1.8205	1.824	1.828	1.8355	
	             1.821	1.824	1.8285	1.8355	
	              1.821	1.8245	1.8299	1.837	
	             1.8215	1.8264	1.8295	1.8375	
	             1.822				
	             1.822				
					
Measurement 	1.820	1.823	1.828	1.834	
Average					
Difference 	0.014	0.011	0.006		
between fired and unfired 					
					
					
Using "D4000' Stoney point gage				
				Indep 	
	Swiss "T"	Norma		Fired	
					
	3.795	3.794		3.8	
	3.7955	3.7945		3.807	
	3.796	3.795		3.808	
	3.7965	3.796		3.8085	
	3.797	3.8		3.809	
	3.7975			3.812	
	3.798				
	3.7985				
					
Measurement Average	3.797	3.794		3.800	
					
Difference between fired 	0.003	0.006			
and unfired

 

[Canuck-IL]

Excellent data!! Thanks for taking the time. I'm not too hopeful that Hornady will answer the e-mailed question. What surprises me is I haven't been able to google up the exact datam line value anywhere online.

The LNL gauge set should be here Wed so I can begin the same measurement process - 3 K-31s with matching bolts and a stack of Prvi fired brass with some unfired Norma and one other brand I can't recall at the moment.

I'll post when I get some results.