BAM 50 : Day 5 (got the chronograph working)

[Ribbonstone]

DAY 5:

Got the chronograph working.


Not sure what was wrong, but as it didn’t work, figured there wasn’t much harm I could do to busted, so I took it apart.

Was hoping to find a dead bug, spider web, or something causes a short inside the screens.  Didn’t find anything obvious, but by cleaning the little circuit boards and un-kinking one of the cables, it started working again.


So how to test it to be sure the readings are reliable?

Broke out a couple of  airguns that were last chronographed,  a Daisy 717 and a gas ram springer.  They read within 1% of the last averages, so figured the chronograph was working.

In an earlier post, with9ut chronographing had estimated the shot count to be something between 35 and 40 shots.  Also guessed by penetration that it was shooting those 8.4gr. pellets considerably faster than 875fps.

For not having a chronograph, wasn’t too far off on my shot string guess.  Rather than 40-45 shots, more like 35-40.  The guess about increased velocity was confirmed.



Actually, this is too easy. All I’ve done:

1. Fix a leak by tightening the nipple.
2. Remove an obstruction to the transfer port (a "mystery" o-ring that shouldn't have been there).
3. Reduce the factory striker spring by 1½ coils and polished the ends.   

One sign of a PCP being an air-hog is that heavy weight pellets earn a lot more energy than medium weight pellets.  Have to expect SOME increase, but in this case it’s a whole lot of increase (18.7 to 23.8 is a whole lot).

I will go back in and reduce the spring guide’s weight to see if I can slow it down a little bit and make the energy difference between heavy weight and medium weight pellets closer.

Need more tests, but it works out to be surprisingly efficient with the heavy weight pellets.  Did work out the numbers for the heavy weights, but I’m going to have to re-shoot a few strings before I really believe them.


THINGS YET TO DO:

I will go back in and reduce the spring guide’s weight to see if I can slow it down a little bit and make the energy difference between heavy weight and medium weight pellets closer.

Stock is still about ½” too long for me.  Butt pad is pretty thick, so will probably have to fit a thinner one to get the length to fit me (and even then, would be a touch long for use with a thick heavy winter coat).

[SpiralGroove]

Hey Ribbonstone,
Looking to see how your stock chop works out.  I'm probably going to cut at least an inch of one of my B50's so my son can use it.
Was thinking I would just mask off the butt and put her carefully under the Radial Arm saw (holding it tight).  A nice, steady cut should do it

[Busta Cap]

Keep the tips coming. Got my two synrods and one Mrod tuned-now starting on my BAM50 .177.  I'm a newbie at this but my BAM is shooting hot. I'm using JSB 8.4's which it likes but they are hitting well over 1000fps consistently, Which is great at 50 Yds but closer in  and the accuracy suffers. I'd like to tune it closer to 900's.  Any advice?

[SpiralGroove]

Hey Busta,
Your problem is too much Hammer strike on the valve stem.
The very quick, cheap route would be to replace the OEM hammer spring with the Hillman #152 -> $2.50 at a local hardware store.
This alone with the stock set-up should drop things down in a jiffy, or cut a few coils off the OEM spring.

If you are accurate at 50 yards, you should be at accurate at 25 yards, the POI will be different, however, because of the trajectory (arc) of the pellet.

[Ribbonstone]

Only really got into tuning two of them ( a .22 and a .177).

Reducing the striker spring will slow it down, but will also lower the operating pressure.  A replacement spring might be a good idea.

Both the .22 and the .177 run at about 2500psi-1700-1600PSI when the isse speing is shortened 1 ½ couls (a little more, as I made a dead end and polished the end flat).

The .177 shot well, but was still a little fast with 8.4’s at that spring length.  More like 1000fps when I’d prefer to stay at 975fps-950 fps with that pellet.


So I lightened the spring guide/top hat a little.  This lowers the strength of the hit to the valve without changing the cocking effort (and at 1 ½ coils shorter, the cocking effort was noticeably less than out-the-box).

PROBABLY could have gotten to 975fps by changing the stoke.  There’s a screw on the front of the striker, and if that screw is moved FORWARD, then the length of stroke is decreased. That would have decreased the operating pressure a bit, but likely could find a setting that gives the “right” speed.

An alternative way of lowering speed without doing any of the above things is to strangle off the transfer port, but you’d have to make a new smaller sized port.  Not something you can buy, but  if you can make one, would serve to decrease the air released at the shot without changing striker weight/spring tension, or anything else.

Another alternative is to try the “bstaley” o-ring tune. (although I haven’t tried it in a BAM50/51).  Here a stack of 0-rings, just larger enough to need to be pushed into the air tube, is stacked between the striker and the valve face.  Idea is that once a certain stacked height is reached, the distance the striker can depress the valve stem is limited.  That would decrease the valve opening, and lower speed.



OR

You could just buy some 10.3gr. JSB’s or 10.6gr. Crosmans.  The increase in weight will be worth something like 50-70fps less speed, which would put you closer to the speed most think of as ideal (although I start to wonder about that).