Curiosity and the Proverbial Cat.
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This has been gnawing at me for a while now- to the point of an "itch".
First, a word from the safety dept:
I don't recommend anyone doing this modification. The stock CO-2 cap is a bit on the short side for this application. The web at the bottom of the plug is just slightly over 1/8" thick. Definitely not to be used with high pressure. Anyone considering this should get either the brass bulk fill cap, or the factory bulk fill cap- both available from Eric at scopes and ammo.
I had a bit of concern getting this set-up past the vent hole in the stock tube. Rightly so- it's gonna be a bear to get off...
After all that- it works! :icon_mrgreen:
I was very leery of that thin web at the bottom of the plug- enough so that I wrapped the rifle in heavy burlap while filling it. I let it sit in the burlap 20 minutes after filling, to make sure.
Results that I wasn't quite expecting with such a small volume. The valve has not been touched on this 78-D. It still has it's piercing pin, and cigarette filter in it.
850 PSI, with the 14.5 gr RWS Superdome:
1.) 519.1
2.) 513.6
3.) 500.0 :highly_amused:
4.) 481.8
5.) 469.9
6.) 440.8
7.) 428.2
8.) 413.4
9.) 393.1
10.) 374.9
Test halted. I didn't want to burp a superdome so it was stuck in the bore.
1500 PSI with the same pellet:
1.) 654.9
2.) 644.8
3.) 634.7
4.) 635.5
5.) 629.2
6.) 624.1
7.) 615.4
8.) 611.8
9.) 607.8
10.) 599.9
11.) 588.9
12.) 582.1
13.) 572.4
14.) 557.3
15.) 544.9
16.) 531.6
17.) 514.6
18.) 499.7
19.) 483.0
20.) 472.8
21.) 446.5
22.) 431.7
23.) 414.2
24.) 400.4
With either/or/and a moderate hammer spring clip/port restrictor, I'd have me a fair to middling indoor sub pal plinker in .22 with this. I'd expect even better in .177, which I'll get around to, sooner or later.
A few important points to cover:
The stock tube on the 78-D is heavy on the inside with machine and tool marks. I still think the 3K tube is the way to go. I had a bit of apprehension taking that stock tube to 1500. FWIW, the inside of the stock AR-20-78-A tube is much cleaner- almost as if extra attention was paid to it...
I'll be discarding that modified tube cap. I don't have that much trust in that kind of steel- it crumbled a bit under the drill, and under the NPT tap. I think for what it was made for- 2 CO-2 caplets- it's fine. Anything else is sort of "asking for it".
Regards,
Doc Sharptail
