A little technical, but hopefully interesting...

[keithherrington]

Over the last three years I've been building and tuning a rifle that specifically shoots 7.5X55 Swiss GP-11 ammo, which as some of you may know is extraordinarily high quality government ball ammunition often found for as little as $.50 a round. The basis for the rifle was a Model 700 action, a Pacific Tool and Gauge bolt, Xtreme CS-22 trigger, three pre-fitted Brux 28" medium Palma contour barrels, and a McMillan Baker Special stock with thumbwheel cheekrest, Anschutz rail and Tubb three-way adjustable buttplate. The scope is a Leupold 6.5-20X50 LR with their Windplex reticle, and a CDS elevation knob calibrated to the cartridge trajectory.

From the beginning the gun has demonstrated two characteristics: extreme accuracy (sub 3/4MOA under all conditions), and an aperiodic tendency to misfire. On each case a second strike of the primer set the cartridge off. This would happen 2-3 times out of 10, which I just didn't care for at all.

My new gunsmith, Dale Woolum of Charlottesville, VA addressed the problem, then took it out for a test run. He shot it a total of 17 times without a hitch and got the following target...at 300 yards. Velocity averaged 2666fps, about 86fps above a K31's nominal velocity, with an E.S. of 43 and a S.D. of 13.7.

What Dale determined the problem to be was marginal firing pin strikes. The firing pin was just too light to work consistently with hard GP-11 primers. He decided to make the firing pin heavier by almost 16%. Original weight was 59.18 grams and the new weight with the tungsten collar was 68.53 grams. Below are pictures of the collar and the firing pin assembly all put back together.

The last picture shows the result of the extra weight. From top to bottom: GP-11 hit by the original weight pin, GP-11 hit by the new heavier pin and a reloaded round, hit by a K-31 firing pin.

Dale makes heavier firing pins for a variety of bench rest gun and precision rifles and in each case he claims to get more consistent ignition and accuracy.

Keith

 

[5-Shot]

Inquiring minds want to know...Would a stronger spring have accomplished the same thing more simply? Is the primer indention a function of firing pin energy or momentum? Way too long since my last physics class.

Ed

 

[LVSteve]

Nice shooting. Is there a tiny hint of vertical stringing in that group? If the holes at the bottom are the first shots, it looks like it warmed up then settled.

I'm jralous of your ammo hoard I have some Swiss rifles that can help out if it becomes a problem.

 

[keithherrington]

Inquiring minds want to know...Would a stronger spring have accomplished the same thing more simply? Is the primer indention a function of firing pin energy or momentum? Way too long since my last physics class.

Ed

I asked him the same question and got a firm no. The lighter firing pin has a tendency to bounce off the primer cup, whereas the heavier pin drives without bouncing all the way to the anvil. The stronger firing pin spring also has the negative attribute of increasing bolt handle lift weight upon cocking. He also talks about the heavier pin settling out unwanted vibration during the firing cycle, but when he goes that deep, he tends to loose me.
Keith

 

[keithherrington]

Nice shooting. Is there a tiny hint of vertical stringing in that group? If the holes at the bottom are the first shots, it looks like it warmed up then settled.

I'm jralous of your ammo hoard I have some Swiss rifles that can help out if it becomes a problem.

The low three are the last three. He and I discussed its cause which I've seen on the range at 600, where after 10 or 15 shots I've got to increase elevation to compensate.

These are two typical 600 yard targets at Fort Meade, MD (shot on two different days) where the wind and light shifts are always challenging. You'll notice that there are two distinct elevations at work, one about 1/2 MOA higher than the other. The V ring is 5" in diameter. A 1/3 MOA shift is what the 300 yard target in the post above shows, but that was with no wind or light shifts to speak of.

We both believe the beeswax sealant around the mouth of each round is the cause. I've been cleaning the gun like any other I have: Hoppe's #9 and a copper bore brush. Since it uses cupronickel bullets (their silver in color) copper plating of the bore is not an issue. He thinks that as the bore gets treated with beeswax it reaches a point (about 10 or so shots) where it is fully "broken in" and then settles into a new group. I'm going to change my cleaning regimen to more closely match the way the Swiss do it: scrub the bore with a nylon brush covered in Automattenfet grease and leave it alone, then wipe it out carefully before shooting it again. We'll see what happens. Remember, it's the journey, not the destination that counts.

Keith

 

[5-Shot]

Thanks Keith. I learned something today.

Ed

 

[JohnRippert]

Best target I ever put holes in was with a K31 and GP11 ammo. I only have about 3 and a half cases though.

 

[bamabiker]

Really nice rifle and shooting.
Beautiful quilt.

 

[loc n load]

Glad you got the ignition issues resolved, nothing more aggravating than a rifle that does not go " bang " when it should, particularly a precision rifle. Impressive rifle and very impressive group as well. Good to know that there are still competent gun smiths out there, having a supply of ammo and spare barrels is simply prudent IMO, not crazy. I will be interested in the effect your new cleaning regimen has on future group consistentency.