Sportered 1917

Thanks for your insight, Patrick

So the GP-11 "is most certainly NOT 0.308"; you've measured it at .3079. Is that a difference of .0001? I won't quibble with you about that .0001 difference in our measurements, since it is with-in the tolerance used by Sierra in the manufacture of their bullets.

I measured it at .308, although it was at the widest point and I was not able to measure the distance along the projectile that maintained that diameter. If, you want to call the point at which the bullet mic's .308...I mean .3079.. a "driving band" and not the bearing surface, that's OK with me.

CarteachO, a poster on a number of mil surp forums and a prolific analyzer of things milsurp, did an analysis of the GP11 and found the diameter of the projectiles to range between .3076 and .308, so I guess both our measurements are with-in the range that he found.

Carteach0: 7.5x55 Swiss GP-11, weighed in the balance

Your specs of the bore/land difference are interesting. If correct, they confirm what I wondered about Swiss barrels--that is: are the lands "taller" (or the grooves "deeper") than US .30 caliber barrels. Your figures show a bore/land difference of .0103, for a groove depth of .00515. That is some what less than figures that I found subsequent to my last post, but still considerably more than the US spec groove depth of .004.

That seems to indicate that , as you posted, the Swiss barrel would take a lot of firing before the grooves were worn beyond the US barrel specs.

Thanks again,

Steve

Before considering the measurements, please take note that I am using a Moore & Wright bench micrometer with a fiducial indicator. It has 0.0001" divisions on a 2 1/2" diameter drum! So if you want to check my results, there is no point in using a $10 supermarket bargain-buy. And when measuring to 0.0001" you need a constant temperature and spotless surfaces.

I checked up on the Swiss measurements. The GP11 bullet appears to be tapered !
Max. diameter at back end (what I call the driving band section)
Today, 0.3080", sitting on my desk in the office. 0.0001" more than yesterday. Bullet was at office temp. Mike was cooler, from the cellar workshop.

1/2 hr later. 0.3079 " again - see what I mean about temperature?
Just behind (0.5 mm) the crimp groove:
0.3045"
At front of crimp groove:
0.3030"
2mm further forwards:
0.3003"

This is a taper of 0.0076" (0.193mm) in 7mm. Approx. 1 degree 35 minutes included angle.

I would be grateful if someone with first-class measuring equipment could check this out, but it seems to me that the GP11 bullet is deliberately tapered to ensure low-shock forming in the forcing cone when the bullet is driven by the powder charge.

These measurements confirm that the GP11 is NOT a 308 bullet as typically understood (i.e. with a more-or-less cylindrical section). And my G96/11 with the GP11 ammo is the most accurate "off-the-rack" service rifle/ammo combination I have ever experienced, bar none.

Patrick

The bore dimensions quoted are CIP, subject to my typos. If SAAMI says different, that's just too bad for SAAMI.

The RUAG Ammotec "Wiederladen" manual (which includes the dimensions quoted) has words to the effect: 308 will work in the Swiss, but 307 would be more suitable. And since that is the Swiss concern that makes most of what the Swiss use, I am prepared to accept that they know what they are talking about.

Patrick

Quote:

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Originally Posted by Patrick Chadwick

The bore dimensions quoted are CIP, subject to my typos. If SAAMI says different, that's just too bad for SAAMI.

Patrick

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Patrick, I have no idea what you are driving at .

Sorry Calfed, I was getting too involved again.

Summarizing:
The GP11 cartridge is amazingly accurate in the G96/11 (also in the K31, according to reports).
The bullet is not cylindrical, but shows a distinct taper.
Measuring the taper requires some care.
This taper could well be an important accuracy factor.
Commercial 308 bullets will work, 307 could be better.
I wish I could get some of those GP11 bullets and try them in a "normal" 300/308 rifle.

And thanks for the link - a very interesting contribution.

Patrick