GTA
All Springer/NP/PCP Air Gun Discussion General => Vintage Air Gun Gate => Topic started by: Jirushi on November 22, 2021, 02:51:50 AM
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I wondered if other people have experienced this:
the piercer cap of the Crosman 400 sealing the front powerlet?
I was using a 400 cap in my Taiyo Juki Bobcat, to use it with 2 instead of 1 powerlet, and each time, velocity drops after about 25 shots, as if I were only using one powerlet, to then rise again for another 25 or so good shots.
I presume that pressure from the powerlet at the valve side, pushes the powerlet at the cap side tight against the cap, and CO2 doesn't really flow past the piercing pin.
The following observations lead me to think that Crosman might have experienced this issue as well:
I have two 400's, an early and a late type, and they have different piercer caps.
The older type has recesses at the bottom of the cap; I think they are meant to make release of gas easier.
I have also seen piercers that look like a screw; probably the threads are meant to make release of gas easier too.
At some point, piercer caps were changed to a design where you have to move a leaver or punch a button to pierce the powerlet; this eliminates the risk of sealing the powerlet by the cap.
Cheers, Louis
(https://i.imgur.com/WCgfK3W.jpg?1)
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I recall having a same problem with my QB78 a while back. I'd been inserting the cartridges, tightening the cap, then cocking and firing it. It turns out I was doing it wrong. Here's what the manual says. It just might work with your Crosman.
Cheers,
J~
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I recall having a same problem with my QB78 a while back. I'd been inserting the cartridges, tightening the cap, then cocking and firing it. It turns out I was doing it wrong. Here's what the manual says. It just might work with your Crosman.
Cheers,
J~
Thank you Goose!
Indeed, I use this technique:
screw cap tight, then turn loose about a quarter turn to release gas, then shoot to pierce the powerlet at the valve side.
This works well in my 400's, but not in the Taiyo Juki Bobcat that I am using with the Crosman 400 cap.
I am probably tightening the cap too much; will try a looser approach.
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Leaving it a little looser sounds like a good plan, Louis. Good luck!
Cheers,
J~
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Louis, a couple ideas:
1. I'd have expected the T-J would have metric threads on its tube, close but not identical to the Crosman's inch ones. If so, this may complicate knowing when it's tightened just enough to pierce.
2. You might try installing the cap to "just barely tight" with two empty cartridges in place, then mark that point with paint to know how far to go with full ones before backing-off. Clearly there's some variation among cartridges, but it would give a reference.
Don R.
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Thank you gents!
@Don R., those are good ideas.
Before going to work, I tested the gun without turning the cap 100% tight (and then unscrewing about a quarter turn). Instead, I felt for the point of resistance, pierced the powerlet, and then unscrewed a quarter turn. The escape of gas was audible. There was a power drop after about 10 shots, but not as bad as before. So, this helps a bit.
Another thing I tried, was to put a piercer in the delrin spacer. With a Crosman 150 cap.
I forgot to mention that the TY needs a spacer as the original cap reaches into the tube quite far; about 1 cm more than the Crosman 400 cap.
This piercer in the spacer was sealing the powerlet too. So I made a trough in the piercer, with my Dremel, to let gas flow freely.
This finally works. 40 consistent shots at 8.4 ft/lbs with 14.35 gr JSB.
Not great (my QB78 does 40 consistent shots at 10.7 ft/lbs).
But "useable".
The bore of this Bobcat looks rough by the way, but I tested it at 20 meters and the groups were good.
Ps. I believe that JW652 has a much neater solution to use his Bobcat with 2 powerlets!
(https://i.imgur.com/IS1Jlxr.jpg)
(https://i.imgur.com/hWGCk4j.jpg)