Swiss bore and GP11 dimensions
Swiss CIP values are:
bore/lands 7.51 mm (0.2957"
grooves 7.77 (0.0.3060)
I just dismantled a GP11 and measured it with my bench micrometer.
The diameter in front of the crimp groove was 0.3030"
The diameter behind the crimp groove was 0.3079"
The diameter is most certainly NOT 0.308" over the full bearing length.
Instead, the short section BEHIND the groove will act as a driving band.
CIP gives the maximum projectile diameter as 7.78 mm = 0.3063"
So, while the rifles may well fire 0.308 bullets without complaining, this will surely lead to a higher chamber pressure and bore wear than that caused by the GP11 cartridge.
But the Swiss barrel will have to be very worn indeed before it loosens up to 300/308 dimensions!
Patrick
A wee bit off topic, but of general reloading interest
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
P.S. to previous message:
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
