Bolt probe Diameter

First a bit of back ground....I am in the process of building a pumper Air rifle, I had a bunch of misc. parts left over from a lot of Air rifles purchased on Gun Broker (20+ in the lot) I have put together and sold most of the complete Air guns, now I have some left overs begging to have something done with them.

This is the build as it stands right now:

Crosman 140 pump tube and linkage.

Valve is a combination of 1970ish Crosman 760 and 2100...

Trigger and sear from a 2100.

Hammer from an early 1377.

The barrel from a 2260.

I fabricated a block to hold the trigger group and a flat top piston. Next I'm starting on the breech....

So far everything is working, the pump pumps and the hammer releases the charge when the trigger is pulled

I'm starting on the breech today and started thinking about the bolt and a question came to mind.....

What size (diameter) should the probe that pushes the pellet into the breech be?

Option 1, a small diameter probe to let the maximum amount of air push on the pellet. Maximum surface area of the pellet exposed to presure from the valve.

Option 2, the largest diameter possible to fill the empty space behind the pellet. With less dead space behind the pellet the air leaving the valve wont have a chance to expand and will expand the pellet skirt and push the pellet down the barrel with maximum pressure and velocity.

Opinions, thoughts?

All my pumpers use option #1.

I was looking at the bolt from a Crosman 1400 and it is a hollow probe that pushes on the skirt of the pellet instead of the center. I like the fact that it would seat the pellet skirt the same distance from the transfer port no matter which pellet is used and I think I may end up starting with a bolt of similar design.

Thats a good point. Pellet skirt distortion from high pressure may not be an issue with that gun/setup but it might be more consistent with the 1400. My Air Ranger is also like your 101 but it is pushing the top accuracy pressure limit for a JSB pellet. It is the most accurate airgun I own but has other reasons I guess.

I got the breach and barrel mounted up and tried a few test shots, getting just under 600 fps with 10 pumps I mounted it in my bench vice next, lined it up with my pellet trap and got an OK group with CPHP. Tried RWS Super Domes and stacked 3 pellets. Next It's on to the stock.....

Cool. What distance?

The groups were shot at 10 yds, in my basement range. Hope to be posting more soon. I have some finish work to do and some testing to do, it worked so well clamping the gun in my bench vice I may end up doing some more testing with the bolt probe before I get the stock mounted.