Tuning Can a 2004 Benjamin 392PA handle 10 pumps?

[seehome]

The new Benjamin 392S and 397S are rated for (10) pumps according to Crosman. There doesn’t appear to be any change to the gun internally whatsoever compared to my 2004 vintage 392PA. Question for the forum: Can the 392PA consistently handle (10) pumps? If it can, it turns a rifle that does 13-ish FPE on (8) pumps into a high 14 to low 15 FPE rifle at (10) pumps.

I’m sure I’m missing something, so what am I not considering? Is this marketing hype from Crosman or was (8) pumps not the real limit for the cartridge style valve?

 

[Beezer]

It would depend on your altitude. I live well above sea level, so I can apply more than the sea level recommended pumps. You need to check velocity with a chronograph. I achieve around 15 FPE with my .20 cal Benjamin Sheridan with heavy pellets at 15+ pumps. It would take fewer pumps at lower altitude. Too high of a pressure will actually reduce velocity in a PCP rifle.

 

[seehome]

I’m about 800 ft elevation. I’m getting an average of 608 FPS with 15.89 grain JSB Hades Diabolos at (8) pumps with my 2004 392PA for 13.05 FPE. At (10) pumps I’m moving the same pellet at an average of 655 FPS for 15.14 FPE. The pumping effort doesn’t feel like I’m straining the gun. If the rifle will do 15.14 FPE with no modifications I think that’s a pretty hot shooter given the platform. Is there any reason to attempt any hot rodding? 

 

[biohazardman]

Seldom anything wrong with a little hot rodding in my book. You just have to remember the weak spots in the gun as you are doing so. Much more pressure and you will begin to stretch the metal at the pivot pin hole on the tube. A common side effect of hot rodding most pumpers. So, a bigger pin and a tighter fit with the tube cap is in order. An extra screw can be added through the tube and into the cap on each side as well.

 

[seehome]

I agree a little hot rodding can be fun - It’s the main reason I bought a chrony. I’ve seen mods to the pins so they wouldn’t wallow out the pump tube, and those make sense with really hard pumping effort. What I’m wondering is whether or not the 392PA platform was always capable of consistently being pumped 10 times without damaging the rifle. Since Crosman is marketing the 392S as capable of 10 pumps maximum using exactly the same cartridge, pump tube, piston, trigger group etc. then it seems to me it should be safe in a 2004 392PA also. 


My 2014 397PA shoots a 10.5 grain CPUM at 755 FPS on (10) pumps giving 13.79 FPE. At (8) pumps that rifle produces 10.1 FPE with the same pellet. If I can shoot that rifle at (10) pumps and not ruin it then that’s good enough for me with that particular rifle. If this information gets out there will be many fewer pumpers getting the “steroid” treatment

 

[Wrongside]

I also have an '04 392PA that I just resealed last summer. Currently I'm getting 620 fps with Benjamin 14.3 HP or about 1 FPE less than yours, but I recently discovered that mine is retaining air starting at 4 pumps. The fix is to either get a stronger hammer spring or soften up the valve spring (so that the existing hammer-spring can keep the valve open long enough for full air dump), or both. If I can get a full dump up to 8 pumps I think I would be able to get closer to the 640+ fps (for 13+ FPE) I've seen mentioned in other posts/forums.

I would worry about accelerated wear on the pivot points for the pump linkages if pumping past the recommended 8 pumps. Guess time will tell if yours can handle the additional pumps.

 

[seehome]

Either the pivot pins will wear out or my joints will - I’m guessing it’ll be my joints that go first

I’m interested to see what the platform can do in a basically stock configuration while maintaining accuracy. I’ve noticed the accuracy suffering a little with the added velocity, even though the gun dumps the entire charge at (10) pumps. My groups open up from about 3/4” at 30 yards (which is about as good as my eyes can manage with the peep sight) to around 1.5”. For the shooting I do 600-630 FPS is fine as long as I’m accurate, and 3/4” groups at 30 yards is still “minute of squirrel” accuracy. 

I also have a 1965 Sheridan Blue Streak which is my current favorite and the best shooting pumper I own. I’m planning to put a Mac1 adjustable piston rod that just arrived today in that gun to try to squeeze out a little more velocity. She currently shoots around 625 FPS using 13.73 grain JSB’s. I’m hoping to get her as close as possible to the 700 FPS mark. I’ve heard that some stock Sheridans can shoot that fast. I’m trying to get there without going full steroid. I’ll post the results of the experiment once I’ve changed out the piston for anyone who’s interested.