Air rifle tuning, the next level….


I had another afternoon to myself today, so I turned my attention back to short stroking Mk3 TX’s and prosports. Their 99mm stroke is lazy and although swapping the heavy steel tophat for one of my lightweight ones speeds things up considerably, its still not that efficient.

I had been thinking that an extended latch rod is the way to go. But with my main priorities being an item that anyone with the most basic toolkit can fit at home for the cost of a couple of tins of pellets, added to the risk of a “hardened” rod failing in the future and surgically removing someone’s thumb, it seemed a piston extension was the way to go! I decided on 13mm as a starting point, reducing the stroke to 86mm with a swept volume of 42.2cc. Slightly over the ideal, but a length which would still engage the anti bear trap mech.

The next question was which material to use? As keeping the moving mass at a minimum I stuck with aluminium and Delrin as test pieces. They are identical in size, but the Delrin has half the mass of the aluminium, so that’s what I’m testing!
Prior to fitting, my Prosport was putting 7.9grn express out at 11.3fpe. With the Delrin extension it dropped to 9.9!! Its inevitable that power will be lost when the volume is reduced, so that was expected. I also tried the aluminium nose to see what effect the extra weight would have, and it went to 10.1fpe. I put the Delrin one back in and added a power washer that brought it up to 10.5 which is where its staying.

I’m struggling to see a downside to using Delrin in this application. The stroke isn’t short enough for any slamming to take place, and used in conjunction with my TbT Drop-in set, the firing cycle can only be described as very sweet! Its a club meet on Sunday, so I’m gonna do my best to pass it around on the zero range and try and get two tins through it before stripping and checking for signs of wear or damage.
It may be that I will have to start experimenting with stiffer springs to get the power up for the shorter barrelled models, or maybe reduce the extension to 10mm, but its been an interesting afternoon!