Air Gun Home :: View topic - BAM B-51

rsterne · Moderator

IMO, the way the 50/51 is set up from the factory is better suited to .177, particularly if you tune it for less than maximum pressures.... Going my route is stressing the gun a lot, and I'm now working on trying to reduce those loads because I don't think it will last doing what it's doing now....

Bob

garththomas · Veteran Member

Thats the problem with these airguns, having and acquiring more becomes addictive and then taking them apart and tinkering comes next and a person may never be happy again. Even though these numbers seem great they are not that far off what my rifle is doing now and since I don't have the equipment to follow along and do more I think I can learn to be happy with what I have. Mr. Green

rsterne · Moderator

Today I pulled the valve apart again, and worked on the poppet and spring.... I ground the ends of the spring flat, which decreased the length by 0.030".... I made a washer that reduced the preload by 0.095", so I shortened the preload by 1/8" from 0.200" to 0.075".... I had a spare poppet, so I tapered the sides slightly to reduce the seat diameter from 0.41" to 0.36", which doesn't sound like much, but that reduced the force holding the valve closed form 396 lbs. to 305 lbs., so combined with the spring change I reduced the effort to open the valve by 25%.... The changes also meant that the valve could open a lot further without the spring hitting coil bind.... Here is a photo of the old and new poppets, shim, and the spring with the ends ground flat....

I reassembled the gun, and expecting it to need less hammer strike I set the preload to minimum, tethered the gun at 2900 psi, and fired a shot across the Chrony.... 1095 fps with 18.1 gr. JSB Heavies!.... REALLY !?!.... Wow, I guess it was driving the spring to coil bind before, I expected the same velocity at less preload, not another 60 fps!.... I tried all the hammer preload settings, and it really made no difference, the velocity was maxed even at minimum preload.... I stripped the back of the gun apart, took the plastic spring guide I had and turned 0.150" off the preload spacer and tried that instead of the metal one.... Now I could back off the velocity a little bit when I set the preload to minimum.... So, I went digging through my hammer springs and found a couple of likely candidates, and ended up with the ability to at least create a velocity curve.... I took that spring and "set" it by compressing it fully several times, and it lost about 0.1" in length, and I reinstalled it and ran one more test.... The results are in the graph below....

You can see that even with minimal preload the stock spring was too strong.... The new, set spring was still capable of getting the gun to maximum, so I tried a few heavier pellets, with the following results at 2900 psi:

JSB Heavy 18.1 gr.: 1100 fps (48.6 FPE)
H&N Baracuda 21.0 gr.: 1053 fps (51.7 FPE)
EunJin Dome 28.4 gr.: 956 fps (57.6 FPE)
EunJin Point 32.1 gr." 933 fps (62.1 FPE)

This gun was no slouch stock at nearly 30 FPE, but I have more than doubled that now, putting it amongst the highest powered .22 cal PCPs.... I then hooked my Chrony up to my Netbook using Chrony Connect, and shot three strings with the JSB 18.1 gr. pellets, as in the graph below....

At 4 turns out from my maximum preload setting (with the new lighter, set, spring) the string started nearly at the top and I only got 15 shots.... At 6 turns out, with a 3000 psi fill, I got a normal bell-curve 1004-1041-998 over 21 shots, averaging 1025 fps (42.2 FPE), at 1.19 FPE/CI.... and at 8 turns out (my minimum preload) I had to start the string at 2700 psi, and I got 20 shots ending at 1960 psi, 980-1015-979, averaging 1000 fps (40.2 FPE), at 1.31 FPE/CI.... The efficiency is down a bit, but still very good considering the power level.... I really like the MUCH lighter cocking force, it should be a lot easier on the gun.... If there is any drawback to the gun the way it is now, it is that I can't tune it down enough to get the JSB Heavies into the mid 900s.... There are two solutions, one is to fit a weaker hammer spring and drop the fill pressure down, the other is to fit a smaller transfer port and run the hammer spring at about 6 turns out, which gave me the 1025 fps string above.... From what I saw previously with this gun, dropping the power by dropping the fill pressure doesn't buy you many more shots, so I think I will try a smaller transfer port and see if that will flatten out the string.... I think something about 9/64" (0.140") should put me at about 35-36 FPE with those pellets, so I will start at 1/8", see what I get, and then try and work my way up until the gun is averaging ~ 950 fps where I know those Exact Heavies tend to still be accurate....

When I took a look at the diameter of the stock poppet I knew immediately why the B-50/51 had such a heavy hammer.... I figured that making it a bit smaller would ease the loads on everything, but I was surprised at just how effective it was.... The poppet could be made quite a bit smaller still, there is no reason the OD couldn't be reduced down to 5/16", maybe even 0.300", because the throat is "only" 0.240".... Doing that would, I think, necessitate making a much lighter hammer to compensate.... That's not a bad thing, but I have the idea in the back of my mind to convert my gun to a .25 cal, so the ability to flow lots of air isn't a bad thing, and I have the cocking effort under control just fine.... Oh, I also took some measurements on the hammer throw, and it can be nearly doubled over stock.... The way my gun was tuned from the factory the hammer travel was 0.52", and it is now 0.625".... I can go all the way to 1.00" by shortening the nose of the striker another 3/8", not that I plan to.... You can't machine off all of the nose right back to the shoulder, or that shoulder will hit the cocking pin when it rotates downwards when you rotate the bolt, but that is the limiting factor.... Even with that shoulder flush with the back of the side cocking slot, the back of the hammer is still behind the sear, so I see no reason that shouldn't work.... Increasing the travel to 1" would allow an even lighter striker and hammer spring.... You would just have to watch that with the valve fully open (to coil bind) that the sear didn't catch behind the hammer....

Bob

rsterne · Moderator

Last night I made a 1/8" transfer port and installed it, and this morning I determined that 1 turn of preload (from my minimum) gave a great string at the power I wanted with the JSB 18.1 gr. Heavies, so I shot strings starting from 3000 psi with five different pellets from 18 gr. and up at that setting.... I shot them through the Chrony at a target at 20' using 5 shot groups, not taking huge care, but good enough to assess the accuracy of the different pellets to do a "first cut".... The 18.1 gr. JSBs didn't disappoint, both types of EunJins, although still one ragged hole at that range, were nearly twice the group size on average, the Baracudas were in between, and rivaling the 18 gr. Heavies for top dog were the 25.3 gr. JSB Monsters.... Usually they shoot visibly worse than the Heavies, but this barrel appears to like them, more on that later.... Anyway, here are the shot strings, starting at 3000 psi, but only including the shots within a 4% ES, so the heavier pellets are missing the high pressure shots, as usual....

Here is the summary....

JSB Heavy 18.1 gr.: 26 shots averaging 949 fps (36.3 FPE) at 1.31 FPE/CI
H&N Baracuda 21.0 gr.: 20 shots averaging 910 fps (38.7 FPE) at 1.37 FPE/CI
JSB Monster 25.3 gr.: 24 shots averaging 847 fps (40.3 FPE) at 1.37 FPE/CI
EunJin Dome 28.9 gr.: 22 shots averaging 811 fps (42.3 FPE) at 1.48 FPE/CI
EunJin Point 32.4 gr.: 22 shots averaging 772 fps (42.9 FPE) at 1.46 FPE/CI

This looks like a perfect tune for the JSB Heavies, and I have lots of preload available to get the Baracudas (and their cousins the Hunter and Hunter Extreme) up into the mid 900s with the small port.... I was quite interested in what I could get with the 25.3 gr. Monsters, and found that at maximum preload (8 turns in) I was getting 913 fps at 2900 psi.... I didn't test those pellets for maximum velocity yesterday when I had the big port in the gun, but I figured it should do closer to 1000 maxed out, so I pulled the gun apart yet again and put the 0.166" transfer port back in.... I tethered the gun at 2900 psi and added the preload data to the graph from yesterday as below....

As you can see, this hammer spring is not quite maxing out the velocity with the 25.3 gr. pellets, there might be another 10 fps in it before it plateaus with the big ports, but 980 fps works out to 54 FPE, so figure the maximum with the stock hammer spring would be 55 FPE.... However, that matters not, because I had to back the preload out half way to 4 turns in from minimum to get a proper string starting from 3000 psi, and here it is....

I'm impressed.... This is the strongest performance I've had from a .22 cal pellet shooter based on a production airgun, although I know there are stronger guns out there (eg. Condor).... I sure hope this pellet ends up being accurate downrange, it certainly is just making tiny ragged holes indoors.... From a 3000 psi fill I got a total of 17 shots averaging 935 fps (49.1 FPE) within a 4% ES, ending at 2180 psi for an efficiency of 1.23 FPE/CI, which I consider pretty amazing at nearly 50 FPE in .22 cal.... Not only that, but 12 shots were within a 2% ES (19 fps variation) averaging 941 fps (49.8 FPE), from 1950 psi down to 2350.... That would be plenty for a days hunt for nearly anything except ground squirrels, at least for me.... I'm pretty stoked that I have a .22 cal that does 50 FPE with a pellet that looks to have promising accuracy potential, and a good BC for great downrange performance.... It should deliver ~ 28 FPE at 100 yards....

With this thing shooting as well as it does, I'm shelving any plans to change it to .25 cal, at least until I have a chance to find out how accurate it is with these pellets this summer.... Unless I have a lightbulb moment, this project is done, at least for now....

Bob

Bob La Londe · Veteran Member

rsterne · Moderator

Box stock....

Bob

Bob La Londe · Veteran Member

humb · New Member

where i can get this rifle in .22 fac in eu ?

Bob La Londe · Veteran Member

Have you had this out to check accuracy at longer range just yet?

Mine has developed a leak, so I am going to have to tear it down soon anyway. Might be a good time to do some efficiency and power tuning.

rsterne · Moderator

Soon, I hope....

Bob

Bob La Londe · Veteran Member

Well, I finally took my B50 apart. Mostly because all the o-rings have failed. LOL. I did a dunk test on the stripped air tube assembley, and the only thing that didn't leak was the transfer port. LOL.

Now I am weeding through your thread to try and find the essence of what the final work was.

I'm looking for a similar tune to where you shot the 26 shots from 3000 with the JSB Heavies. I guess I'll start by making a list of all your changes, and then lining out the ones you reversed somewhat like preload length and transfer port size.

rsterne · Moderator

The poppet mods (thinner spring spacer and turning down the OD at the seat).... are IMO the most important thing you can do to make the poppet survive, and reduce the cocking effort by allowing you to use a lighter hammer spring preload.... Since it changes everything else, you may as well start with the valve mods, and work out from there.... Be careful on hogging the exhaust port and rounding the bottom corner, or you can break through the valve seat face and/or ruin the seat.... There is room, just use care.... You do NOT want to damage the valve, as the valve body and tube are tapped for the upper screws together, pretty much guaranteeing you can't replace the valve without a lot of work....

Bob

Bob La Londe · Veteran Member

Bob La Londe · Veteran Member

My poppet is radically different than yours. It seals inside the opening of the valve, not on the outside like yours. I don't know if the gun changed or if this is something Mike Melick did before he shipped the gun.

I definitely see what you mean about that spring. Its a total beast. Is that heavy of a spring really necessary? I would think something half that strong would do. I don't think the springs in either of my Marauders are that heavy. Neither is leaking at the moment so I don't want to take one apart right now to compare.

This is a dead nuts simple valve. When I get around to my own ground up builds I think I'll start with a simple design like this.

rsterne · Moderator

May be a Mike M replacement, no idea.... Once the valve seals inside the throat, the spring becomes a much larger part of the closing force, and reducing it will allow a much lighter hammer and/or spring at the same air pressure....

Bob