My results with a Terry C. modified Model 25 shot tube.
The host gun is a vintage Model 99 that has been resealed about 1,000 shots ago, with a Cobalt 327 high powered spring and oversized air tube installed (I think 7/64" drilled hole), running with 20wt non-detergent oil, shooting standard, cheap, bought from Wal-Mart Daisy brand BBs.
These bb guns can be fickle and some of my numbers don't make sense in absolute terms, but I believe there is at least value in them for relative terms.
I am concerned that the 30 degree temperature increase is skewing my numbers, but regardless, this is what I noted. Also, it has been my experience that these BB guns increase in velocity after they have 100 shots through them. I am guessing the seals settle in and the spring rotates to a stable position. Unfortunately the following numbers were the first ~20 shots before I made a change, I am guessing these numbers are low.
When it was 50 degrees outside with a new production, Model 25 shot tube I was getting 317 to 327 fps. This was the set-up that was in this gun since it was resealed 1000 shots ago.
By the time I got around to making a Terry C. style modified shot tube it had climbed to 80 degrees outside.
These are the following variations I did:
-I drilled the front cap and installed the brass feed tube, I did NOT epoxy it at all, just reassembled and put the rubber plug in the front to keep the BBs in. I got right around 305 fps, but when I reinstalled my old shot tube that had given me ~317 to 317 fps when it was colder out the speed had dropped to ~305 fps also.
-Next I tried removing the air tube (I cut an old airtube off flush with the front of the plunger seal to close off the hole in the seal face) so it was more like a model 499. That was a joke, the Terry C. modified shot tube was at 120 fps, however the original shot tube was even worse, it was down to 35 fps. The air tube may kill speed in some ways, but it is very necessary for some applications.
-Next I used RTV sealant to plug up all of the air gaps in the part that screws into the abutment seal, the same way Terry C. used epoxy. This did NOT help the fps without the airtube installed.
-I reinstalled the oversized airtube, with the original shot tube that had been giving me 317 to 327 fps, the velocity was down to 289 to 305 fps.
-Finally, I installed the Terry C. style shot tube sealed with RTV. The speed was 303-315 fps. Also, it feed great, without any issues.
In Summary: the Terry C. style shot tube added ~10 to 15 fps. However, it also made loading the BB gun so much easier and I am guessing it will generally help with accuracy (Since I can wrap the front cap with more tape to keep the front of the shot tube from moving around, and really tighten up the shot tube in the rifle since I won't need to remove it for loading.)
The easy loading function alone, would make this modification worth doing, the small speed bump was a nice bonus. I will run it over the chronograph after I have a few 100 rounds through to see where the velocity goes.
Thank you Terry for the great idea and providing links for the hard to locate parts.
Thank you Cobalt327 for bringing the super spring, bored out air tubes, and seals to an easily accessible marketplace. I blame you for starting my collection of and modification to Daisy BB guns. I had totally written off BB guns before reading your posts.