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

Bob La Londe · Veteran Member

Ok, here is my synopsis of what you did to this gun. Did I miss anything?

1. Clean barrel.

2. De-burr airtube, hammer, breech.

3. Remove striker, and slot back end for easier adjustment through rear of gun.

4. Plastic under striker locking screw to act as brake, but allow for rear adjustment. Two turns out from stock most efficient.

5. Lengthen Hammer Stroke
Shortened front of hammer by 2mm
Made new flush striker. Net gain 2.75mm stroke

6. Reduce Preload
Plastic spring guide that is ¼” shorter in thick part

7. Lighten Hammer
Plastic spring guide that is ¼” shorter in thick part

8. Remove anti rotation screw from hammer

9. PUT BACK ORIGINAL SPRING GUIDE

10. Make rear velocity adjuster.
Bore out and slot rear cap.
Shorten spring guide ¼” to accommodate.
(Looks like fat end of spring guide so its also lighter)

11. Milled Transfer Port Recess to 5/16 diameter and into barrel slightly
New teflon 5/16 transfer port tube has .166” ID

12. Drilled through to chamber with #20 .162

13. Drilled exhaust port to .172 – deburr with round Dremel burr.

14. Added washer to protect filter screen in fill valve.

15. Ground ends of poppet spring flat.
Shortens spring about .030”

16. Made thinner poppet spring washer.
Shortens preload by .125”

17. Reduced engagement diameter of poppet.

18. Shortened plastic hammer spring guide .150 and reinstalled.

19. Installed lighter hammer spring.

20. Installed 1/8” transfer port
(1 turn preload) Near ideal strings.
26 shots with JSB heavies around 940-950

21. Reinstalled .166 port
4 turns plus from minimum
15 shots with JSB monster around 930

rsterne · Moderator

You don't expect me to remember, do you?.... Rolling Eyes

What I wrote in the thread was what I did.... Laughing

Bob

Bob La Londe · Veteran Member

Have you by any chance setup a jig for measuring compression spring force. In this thread it seemed like you just grabbed a hammer spring that felt right.

Sine I don't have a box full of old hammer springs that's not necessarily an option. I have to order springs from somebody like McMaster. They are only a couple bucks apiece, but still.

Bob La Londe · Veteran Member

Bob La Londe · Veteran Member

rsterne · Moderator

I use a piece of angle iron clamped in a vice.... It has a 1/4" hole drilled in the horizontal part, clear of the vice by a couple inches.... I use a 6" long 3/16" screw hook up through the hole.... Slide the spring over the shaft of the hook and put a nut on top.... The hook is hanging below the angle iron....

Measure the distance from the nut to the angle iron (the free length of the spring) with calipers.... Hang a known weight on the hook (I use about 7 lbs.).... Measure the distance from the nut to the angle iron with the spring compressed by the weight....

Subtract the compressed length from the free length.... Divide the weight by that length difference.... The result is the spring rate in lb./in....

Bob

Bob La Londe · Veteran Member

Bob La Londe · Veteran Member

I guess I'll just have to try it and see, with a poppet spring that is actually lighter than stock I may have to use the 10lb spring.

Bob La Londe · Veteran Member

Bob La Londe · Veteran Member

Well I shot it today.

This is what I wound up doing.

1. Cut thinner poppet washer for the poppet spring.
2. Polished a few thousandths off each end of the poppet spring.
3. Shortened hammer spring guide about .30 - .35 (not sure my lathe bit broke.
4. Shorten nose of hammer about 2.2mm.
5. Ground striker flat.
6. Punched out a couple tiny circles from a water jug and placed them under the set screw for the striker.
7. Installed 12.9lb spring from McMaster.

DID NOT TOUCH THE VALVE BODY except for all new o-rings. Since you (rsterne) had dropped back to a .125 transfer port I decided to see what the current setup was first. A .136 drill bit slid easily into the stock transfer port tube. It was a little sloppy in the barrel/ breach hole and in the exhaust port of the valve so I decided to try it the way it was first.

8. Installed .22 breach and barrel. I almost cried at this point. I'd never checked it before. The barrel goes off to the right about 1/4" or a little more over the length of the air tube. Its pretty obvious.

I was headed for an unofficial meet (I was the only one who showed up), so I hurriedly finished it up and brought it along. Figured I'd mount the scope at the range.

With the barrel looking so bad the only .22 pellets I brought were a cheap tin of CPHP 14.35gr. With a 3000 PSI fill the first shot is in the 1055-1058 range on every fill, and slowly drops to 100X on shot 18. Consistently mid 1050s to 1000+ for 18 shots. It has no curve. Well it might have some but not much. For the next 20 shots or more it drops very slow down to 920 ish. I'll looks at my shot strings later. Regulated to about 1400 PSI this gun would shoot 920s for a very long time with that pellet.

Its pretty accurate though. I almost wish it wasn't because of the way it looks, but it is.

If I hand picked pellets with no flashing, burrs, or dents it was a hair high 25 yards and dead on at 50 yards. There was an occasional wild shot when I was just stuffing and shooting, but the gun likes cheap pellets. I shouldn't be surprised. The .177 barrel and breach likes 10.5 CPDs just fine.

After a while I got bored shooting spinners and paper so I started shooting other stuff on the range. Shotgun shells from 30 yards to 60 yards were easy pickings. Then I picked on some golf balls. I ordinarily do not shoot golf balls with an airgun because they return pellets with an amazing amount of energy at short range. Those I picked were far enough out I didn't figure it would be an issue. Then I spotted one on the side of the hill that is the back stop for the small bore range. The first little hill is about 135 yard (I've ranged it before when powder shooting). I had to back the scope out to about 7X to see where the pellet was hitting with some leeway. It took me 8-10 shots to kinda get the range. There wasn't much breeze, but enough to feel it. On a fresh fill I could drop 5 or 6 shots in the proverbial dinner plate or better, but I just could dial it in. Then I topped off the gun one more time and hand picked five pellets. The first shot was dead center and about 1/2 low. Second shot drilled the ball and sent it up the hillside. It rolled back to within a foot of where it was. The third shot rocked it and it rolled a couple inches. The fourth shot sent it skittering up the hill again, but when it rolled back down it disappeared behind a fold in the hill. 3 shots in a row at a golf ball size target at about 135 yards. I'd have to say if I'd hand picked all the pellets it might hold 6 inch groups at that range. Maybe better with somebody else shooting.

After that I went back to drilling shotgun shells at medium range, but the fun wore off after a little while.

Now I have a decision to make. Do I live with a gun that looks like $hit with a crooked @$$ barrel, but shoots pretty darn good, or do I detune it and throw the .177 breach and barrel back on it.

Obviously I need to figure out how to get into the curve no matter what I do. Maybe this weekend I'll try that 10lb Marauder spring. I see no need to try the lighter poppet springs I bought at this point.

~~~~ After some thought ~~~~

I think I'll throw the .177 breach and barrel back on the gun and see how it shoots heavy .177 pellets with this tune and/or with the 10lb hammer. I think I've got a tin of 16 grain rabbit magnums in the cabinet.

Bob La Londe · Veteran Member

~~~~ The Next Day ~~~~

I just pulled the barrel band off today in preparation for changing the gun back to its .177 barrel and breech, and the barrel swung over to be centered over the air tube. Now I am really confused. I'll look it over carefully, but I guess putting a couple hundred rounds through it yesterday relieved some stress or misalignment somewhere. Not at all something I expected to happen. I put the barrel band back on and all is straight. I'll bet I have to resight the gun, but... I don't even have a clue what might have happened.

Bob La Londe · Veteran Member

I setup a proper spring gage today, and weighed the lead bars,I was using. All of the McMaster 12.9 springs scaled in at 12.3-12.5. The original BAM spring came in at 13.5 I found two 10lb springs when I looked thru my Marauder parts. They look totally different. When I checked my purchase history one came out of my .25 and the other was suppose to be one of two 12.5s from DiscosRUS to bump up the power on the .25. If the one in the gun is the same it might explain why I was never able to tune as hot as I thought I should. I guess it's time to pull the hammer spring on that Marauder again and check it now that I have some known heavier springs to try.

Bob La Londe · Veteran Member

I dropped one of the 10lb springs in the gun today. Still haven't made a preload adjuster for it. Just wanted to see where I am at. I've got a particular goal of 26 shots give or take around 950 with JSB heavies.

I'm thinking my way through a different preload adjuster design than Bob Sterne's.

I don't have any JSB heavies right now so I am testing with what I have. Since the 14.35 grain CPHP pellets shot so I well I decided not to waste anymore of them just shooting a string for speed. I pulled out a tin of 14.5 grain RWS pointed pellets. I have not found a gun they shoot well in so they just sit there taking up space for the most part. Sadly I still haven't found a gun they shoot well with.

So with a 10lb spring and no preload other than the minimal amount from stuffing it in the gun I shot 51 shots from 891-978-877. 3000-1500 PSI

I do know from previous testing that these pointed pellets tend to fly a little faster than other similar weight pellets. In my Hatsan 155 the 14.35 CPHP pellets run very consistent 930, and the 14.5 RWS domes scatter in the 940s. I'll assume they are ten or so FPS faster in this gun as well. When I up size to the 18 grain pellets I'll guess my velocity will be at least 30 FPS slower than this curve. Maybe more. I'm sorta guessing here.

As you can see from the data I've got a really pretty bell curve now.

I think if I just wanted to shoot the 14.35 CPHP pellets I already know the gun likes at speeds upto 1058 FPS I could just find the right max fill pressure and call it a day.

I've got 26 shots within 30 FPS and a nice curve on shot 13 thru shot 38.

Now I am thinking about my options and more than willing to listen to some opinions.

I suppose before I can do any more real tuning I'll have to buy some JSB heavies so I can establish a baseline with the pellet I want to shoot.

HEY BOB, You weren't at all kidding about the lightened cocking effort. You barely have to think about it to cock it now.

Bob La Londe · Veteran Member

Well, I checked my points @ Pyramid Air and had enough for a free tin of pellets, so the heavies are on the way.

My thoughts on bumping the power since I am so close is to just bump the preload a bit. Probably test with washers like Mr Sterne. Alternatively I might try a smaller transfer port and look for an 11lb spring.

I haven't looked to see how much room I have to work, but I think I can drill and thread the rear retainer, and make an insert with lip on the outside to retain the safety. The insert can be drilled and threaded for a screw for a power adjuster that can adjusted and fixed in place with a simple stop nut. I can simply drill out the hole in the safety if I need to make the threaded insert a little bigger.

I'm doing a honey do right now, but if I have time I might draw up something this evening.

Spinalmechanix · Member

Rsterne, How was the down range accuracy? That fpe is incredible.