Chamber Size Differences...

[BCB]

The information: Two T/C Contender Super 14” in the 7-30 Waters cartridge…

My Waters barrel is 20+ years older than the Waters barrel of a friend…

I loaded a few rounds setting the RCBS dies to “touch” the shoulder of his fired cases. So, I thought I would shoot a few in my Contender as I had never tried the bullet being loaded for his Contender…

I go to my range and guess what, they would not chamber…

So, I came home and measured the headspace length of a case fired some time ago in my Contender and I measured a case fired in his Contender…

I used a Hornady Headspace gauge and the difference was a whopping 0.017” of an inch. His chamber was that much longer. No wonder his fired and reloaded round would not chamber in my Waters…

Is this acceptable or even within specs of a Waters chamber or any other chamber for that matter to vary that much?…

His Waters shoots well and has had no case separation…

Any thoughts?...

Thanks…BCB

[BK7saum]

Was the older barrel made before the 7-30 was adopted as a saami cartridge. There are typically many variations on chamber reamers for wildcat cartridges. But since it is a rimmed cartridge the distance to shoulder means very little in regard to case head separation. The rim restricts the forward movement of the case not the shoulder.

[Ballistics in Scotland]

Here is the SAAMI specification, which probably hasn't changed since they first listed the cartridge in 1985. It specifies a tolerance of .007in. for maximum tolerance on the rim, which is what matters. But although the length to the imaginary intersection points with the shoulder are listed as reference points (imaginary because the cae-forming process means the intersection is rounded), no tolerance is given for these.

.017in., for this particular dimension, isn't as whopping as all that. The mid-point between the two shoulder intersections is given as 1.6747in. from the breech face. Only on their cartridge drawing is this made a reference dimension, at 1.6576in., with a tolerance of .015 under, but nothing over that figure. This means SAAMI regard an airspace between chamber and cartridge shoulders of .021in. as acceptable.

It sounds as if someone's chamber, while not contravening SAAMI standards, isn't quite what it ought to be. Most likely it is your friend's. But it is harmless and should have no detectable effect on accuracy.

http://saami.org/PubResources/CC_Drawings/Rifle/7-30%20Waters.pdf

[BCB]

Yep, my misuse of the word “headspace”. Rimmed cases on the rim, belted cases on the belt, and rimless on the shoulder…

But, I always try to have the shoulder touch the chamber when I can. Or at least keep it minimally 0.003” away. This is only for some of the most accurate cartridges/firearm combinations I shoot. Others are not measured but chamber—basically neck sized, or thereabout!...

So, with the long chamber in my friends Waters, I set the dies to size them about 0.003” short of the shoulder touching the chamber. They all chamber nicely and the T/C closes with its distinct “snap” sound. Of course at that sized length, they will not chamber, without much effort, in my Waters—all well and good as I won’t be shooting them anyhow…

Any yes, my barrel was one of the first ones off of the assembly line from T/C and that would have been in the late 80’s, I think as I have a fair collection of barrels and the chronological order of purchase fails me. Maybe not to SAAMI specs then? It had to be ordered by my local gun shop. The groups I shot with it were amazing and he made copies of them to show future customers of the accuracy potential of the Waters barrel and the Contender. Said he sold a few just from showing copies of my groups! My friend’s is only a few years old so I suspect it is to SAAMI specs…

Thanks all…

Good-luck…BCB

[Ballistics in Scotland]

I dare say this is irrelevant for the sort of accuracy you really need in a pistol, but with .003in. clearance differences in temperature, springiness of the brass, fouling of the chamber etc. mean that the shoulder is likely to sometimes make contact on the shoulder and sometimes on the rim. It might be better to have it consistently do one thing or the other even if it seems like less precision in reloading.