My Benjamin 392

[rsterne]

Here is the testing with the valve mods so far.... larger port, throat, and MRod poppet.... I am only checking the velocity every 2 pumps.... The MRod poppet and valve spring, sitting on a #8 washer on the shoulder in the valve front, is just about perfect for this gun, IMO.... The poppet is smaller in OD, it has a smaller stem, the gun is easier to cock, and you can still pump the gun without having to cock it....



As you can see, the improvements continue.... In addition, I can now pump 14 times without retaining air.... I didn't try more than that....



I am no longer going to test anything lighter than 14.3 gr. pellets.... and I am thinking about pellet weight in the reverse, compared to before.... Instead of not looking heavier than 19 gr. because the FPE doesn't increase as fast, I think I should not be looking at pellets lighter than 15.9 gr. because the FPE deceases quicker.... Sometimes I get it backwards, and this is one of those times.... 

The gun is now producing 21 FPE at 12 pumps with 25.4 gr. pellets....  …. The next set of mods is to drill out the receiver and barrel port, which is now the smallest port in the gun.... While I'm at it, I will slim down the bolt probe as well.... I think that will complete the "breathing" mods, then I will take a look at the pump.... or, I may install a pressure gauge....

Bob

[Rob M]

very nice Bob.. this thing is gonna be a fire breather.. I think youre already in the steroid realm of power IIrc...

[Hobbyman2007]

Bob ,
Figured I’d reply here instead of on the CAF as there are simply more views here. I just ran a test with my Steroid 392 and figured you’d be interested in seeing what one had for comparison. Here goes. Shooting the 28 grain BBT in .217/219”
445 FPS at 5 pumps
530 FPS at 8 pumps
575 FPS at 10 pumps
610 FPS at 12 pumps
615 FPS at 14 pumps ( retaining air)

As you can see I was slightly off with my previous numbers. My memory is not as good as I remember. Haha I was really expecting to hit 30FPE with it but it’s still no slouch at 25. I think there’s more left in it but the reality is I won’t bother. Pumping more than 10 times between shots is well a bit too much work when I can grab a springer and do the same for less input. I’m happy with the 575fps at 10 pumps and the many grouse and hare it has taken didn’t know the difference.

[rsterne]

You are getting the same velocity at 8 and 12 pumps with 28 gr. BBTs as I am getting with the 25.4 gr. Monsters.... Were you using the HPs or the FN slugs, when you checked the velocity?.... What changes have you made that I haven't done yet?....

Bob

[Hobbyman2007]

Bob ,
I’m not sure what has been done to this particular gun. It is exactly how I bought it ( used) and I have never been inside it. I was using the HP bullets and the were all sorted by weight averaging 27.7 to 28 grains. I was told by the seller that this rifle was done by Mac1 then brought to Canada. I can see that the pump lever is different than stock and so is the adjustable piston rod. Other than that I’m not sure what has taken place on the inside.

[Techie]

Here are the numbers with my Steroid 392 (now Duy's).  This was tested when I first bought it from Timmy Mac in 1998. I have done nothing to it.
Note this was at 7400 feet elevation, so each pump is compressing less-dense air.

FPS     Pellet weight  FPE     Pellet name & # of pumps
750        14.8            18.49   FTS 14 pumps
775        14.8            19.74   FTS 16 pumps
528        21.6           13.37   Beeman Kodiaks 8 pumps
640        21.6           19.65   Beeman Kodiaks 14 pumps
682        21.1           21.80   Beeman Kodiaks 16 pumps
572        29              21.07   Dae Sung 14 pumps
611        29              24.05   Dae Sung 16 pumps

[rsterne]

Denis, my HPs weigh 27.4 gr, possibly a different alloy (I use 40:1) or casting temperature (I run very hot, 800+)…. The Mac1 Steroid uses a beefier pump linkage (I had one in my Millennium Pumper), and an adjustable piston.... I think it still uses the rubber cup, though.... How far from closed is the end of the pump arm when the cup touches the valve?.... Is there a felt ring on the piston for lubing the pump?....

Mike, thanks for the additional data, much appreciated.... I'm guessing the Steroided 392s get a lot of their additional power from a more efficient pump.... and extra pumping, of course....

Bob

[Hobbyman2007]

I don’t see a felt on the piston for lube and as for how much clearance , I think I can adjust it a small amount. I have a felt pad that keeps the arm from slamming into the tube. This is most likely also increasing the headspace a bit. I doubt I kissed that much but might explain why I thought I was getting 30FPE. I’ll check to see if I can make a small adjustment and gain anything.

[rsterne]

The pump linkage on the 392 goes "over center" when fully closed.... so a thin felt pad might actually decrease the headspace a bit.... Before you make any changes, after you take a shot (when the gun is empty of pressure), if you open the pump arm a few inches, and then close it gently (only push it hard enough to move it)…. where does it stop, ie how far from the pump tube to the end of the pump handle?.... That tells you when the pump cup is hitting the end of the valve....

Bob

[rsterne]

Charles, I have a question about the trigger and a possible velocity adjuster or SSG.... You mentioned you have done a trigger mod, could you provide a link, please?.... The stock trigger has a blade at the top that prevents you pulling the trigger when the bolt handle is not "down and locked".... It must rise into the flat on the back of the bolt to allow the gun to fire....

The presence of that blade makes a preload adjuster difficult, and an SSG well nigh impossible.... Since I want to develop my 392 to retain air for a second shot, adjusting the hammer strike is a necessity, and if I can I thought I would incorporate an SSG at the same time.... The "easy" way to do that would be to cut the top off the trigger blade (eliminating the bolt lockup safety feature), and make an insert for the top of the trigger frame that is threaded for the adjuster/SSG.... It looks like the ID of the trigger housing at that point is 3/4", although I haven't tried a piece of 3/4" stock in it yet....

My question to you and others, do you have any information on preload adjusters (aka a power adjuster or a tuner for an ACP/RAP) for the 39X.... Any photos or links would be much appreciated, no sense in reinventing the wheel.... although I doubt anyone has fitted an SSG to one yet.... 

The other thought I had was to try and make a "Dual Power Hammer", like the ones made in Canada a decade ago for the 13XX and 22XX series Crosmans.... It looked like this....



It was cocked from the rear, and the hammer had two sear notches.... You could tune a pumper to retain air for plinking (multiple shots) and then by just pulling back to the second sear notch you had full power, with enough hammer strike to dump the air.... I haven't given this any thought yet for a 392, so I don't know if it can be made to work or not....

Bob

[Hobbyman2007]

Well Bob I checked and as I thought there was still space before the cup touched the front of the valve.  I have it adjusted now at just shy of an inch at the rear of the pump arm at contact. I ran out of time and won’t be able to test for a few weeks. Off for work again. This time 16 days then off for 12. I can’t wait for the off for 12 part. 

[Psipumper]

Bob, I haven’t ever posted any details on the trigger works. I would have trouble with the wording most likely. The interlock can be removed , not needed to improve trigger in anyway.

Adjustment ended with the handle in this position. Mac1 adjustable piston.

[rsterne]

Thanks, I didn't see any issue in removing the interlock.... One advantage is that you can uncock the gun without it, something you cannot do in stock form.... I have never seen any problems mentioned with the bolt flipping open like it can on a Disco, so that will be one of the first things I do to the trigger group....

The other thing that will get my attention is to rework the sear angle so that it doesn't slide the hammer back when you squeeze the trigger.... That should be as simple as making a jig with a pin in it for the sear pivot point, and clamping that on a disc sander and moving it in until the sanding disc touches both ends of the release face at the same time.... Basically, sanding the sear until it is part of an arc of a circle, centered on the sear pivot hole.... I will then check the sear catch surface on the hammer to make sure the angle matches fairly close.... I may fit a lighter trigger spring as well, depending on how the trigger feels after those changes....

After that, I think I will start working on a preload adjuster for the hammer spring.... or maybe an SSG.... 

Bob

[Psipumper]

I used 10 degrees on both hammer and sear.
Try this. Remove the small coil spring inside the trigger assembly. Some people call it a lawyer spring. When you put it together without that spring, feel of the trigger . Now bolt the trigger assembly back on the tube.Trigger still feels good. When you put the hammer in the tube, now the trigger has a click at the beginning of pull. The reason is the sear is too tall where it engages the hammer. When the small amount of material is ground off the sear height, this click goes away.
This is the cause of the creep people complain about with the “lawyer spring “ installed .
IIRC , it’s only about 1/32 to come off the sear. The click goes away and the hammer slides with gravity when you tilt the tube.
Very important not to take too much material off.
The creep is gone and the trigger pull is shorter and lighter. The “lawyer spring “ is no longer used.
The bad with this is the safety tang moves away from the safety block. I welded the tang and fitted to correct this.
I also added an aluminum spacer to stop trigger over travel.
2 lb. trigger smooth safe trigger now.

The Same trigger feel but longer  pull could be got by undercutting the hammer grooves deeper.

Anyone should do this at their own risk and ensure the gun is safe and handle it safely.

[rsterne]

Here is the start of my preload adjuster, which may morph into an SSG.... I cut off the top of the trigger, which formed the safety interlock with the bolt.... This is designed to prevent firing the gun with the bolt open, but also meant that you could not uncock the gun, a feature I really didn't like.... I then took a 1.25" length of 3/4" OD 6061-T6 aluminum bar stock drilled a 21/64" hole through it so that it could be tapped to 3/8"-24NF for an adjusting screw, and then faced the ends off so that it was a drop-in fit into the recess in the top of the trigger housing.... It is a perfect fit, providing you get the length right.... Note that the potmetal casting has a slight taper to allow it to come out of the mould, so you have to measure the length at the bottom of the recess, not the parting face.... and on my trigger housing one side was about 0.005" shorter than the other.... 



I will drill out the ends of the housing at some point when I get ready to make the adjuster, but for the next round of testing I don't want to disturb the spring seat, so that will come later.... Thanks for the advice, Charles, I will look at that when I work on the sear.... I will likely remove the trigger spring completely, and just rely on the hairpin sear spring.... I thought the "click" was from the front section of the sear engaging the forward band on the hammer in "half cock" mode?.... I am contemplating removing that completely, is there any downside to that (besides losing the half-cock) ?.... My understanding is that the "creep" was because the stock sear is moving back and forcing the hammer to move back as well.... That increases the hammer pull if you use a stronger hammer spring.... and why I was going to change the sear angle to avoid it moving the hammer back....

Bob

[Psipumper]

Not sure of the second catch on the sear purpose. I did have to shorten that because the hammer would catch on it when fired because I shortened the main sear catch. I didn’t remove the second catch completely because I thought when the bolt was pulled back it may support the hammer straighter jumping over the sear shape.
The creep felt is solely due to the hammer preloading the excess height of the sear , kinda uncoupling the trigger and sear.
Charles

[rsterne]

There is info on the 392 sear angle from Timmy@Mac1 in this thread....

https://www.gatewaytoairguns.org/GTA/index.php?topic=37553.

Bob

[Psipumper]

There is info on the 392 sear angle from Timmy@Mac1 in this thread....

https://www.gatewaytoairguns.org/GTA/index.php?topic=37553.

Bob

I haven’t seen that thread but I have seen the AOA guide link that was in it.
I can’t remember how I came up with 10 degrees. Most likely I measured the rearward movement of the sear when it dropped and trigged the angle.
I held a 10 degree angle block next to sear and scribed a line to grind to. Did the work with it assembled.
For the hammer 10 degree cut in lathe.

This is a separate issue from the sear height issue.

Charles

[Hack21]

Bob,

Here is another trigger thread that I recently posted if you have interest in it. Hopefully the link works. If my memory serves, the sear engagement is about 0.050", so it is only necessary to change that much of the angle. I clamped my sear to a large framing square and used 600grt paper on my cast table saw top.

https://www.gatewaytoairguns.org/GTA/index.php?topic=166326.msg155860888#sharebuttons

[Psipumper]

Bob,

Here is another trigger thread that I recently posted if you have interest in it. Hopefully the link works. If my memory serves, the sear engagement is about 0.050", so it is only necessary to change that much of the angle. I clamped my sear to a large framing square and used 600grt paper on my cast table saw top.

https://www.gatewaytoairguns.org/GTA/index.php?topic=166326.msg155860888#sharebuttons

Thanks Eric,  you have the correct terminology. What I am calling creep is take up.
I think we end up with the same, just different ways to fix it.

Charles