Brocock Bantam Tuning

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"Skip-in-WV"I noticed the adjustment screw in the pictures is dark and has a hex head. Mine is bright and has a slotted screw. Did you make yours? What size andthread is the screw?
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Both of my screws are different lengths, but each is stock, black and has a hex head (even though in the pic you can't see it) The head is inside the hammer and is adjusted by putting a long hex thru the tube end cap while assembled. If you are referring to the slotted "screw" with the hole in it on the back side of hammer that is what holds the hammer inside the cage. It also contains the spring and cushion. You put the hex wrench thru that hole to adjust the stroke. If not please post a pic of what you are referring to.
 
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A couple of notes - make sure that you re-tighten the set screw on the side of the hammer to prevent the hammer screw from walking back in. I had that problem over the summer - could not for the life of me figure out why I was loosing velocity... I did wind up figuring that out and make double sure I tighten it back up each time now.

For the SSH, I don't think that the hammer screw needs to be all the way in, so only the inner hammer hits (that was my 550 fps problem). Just that when the outer hammer hits the shoulder, then the inner hammer needs to slip forward a bit, continuing to open the valve via momentum. Then when the valve returns, the inner hammer retracts via its spring and the outer hammer does not move with a heavily preloaded or strong short spring. I might try the original spring again to see what the efficiency is now. One thing is certain, the factory does not "tune" the rifles for efficiency off the production line. 

I would agree that 0.95 efficiency is not good. I one time got 1.7 with my Compatto 177, and 1.3 from this rifle - just don't know what the secret formula is to get it back there. One other possibility is that the pressure is decreasing as a result of cooling of the air in the tank as I shoot. I am filling at 65 deg in the basement (tank is really warming to 80 deg-ish via the filling process, which other folks may see as well), then shooting out of my garage at 25 to 30 degrees. Even in my 10M shooting cellar, it is 45 degrees. So maybe some of the "air use" is really from the temperature decrease, hence the efficiency appears poor, but really is "ok". I did some math 80 deg to 30 degree gives about 10% pressure loss - back fudging the pressure numbers to account for temperature swings, indicates that real efficiency might be around 1.22 fpe/ci. I would still like to get it more efficient tough. I am hoping for some warmer weather to shoot outdoors. I also just ordered some of the redesigned JSB 13.43 177 pellets to see if they group better at 55 yards as well.
 
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Does this picture help withe the adjustment Skip? The slotted retaining screw is only used for holding the hammer in alignment in the receiver.

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Here's a write-up from Airgun World at the release of the Compatto:

http://www.brocock.co.uk/reviews/AGW-1215-Compatto.pdf

"SLINGSHOT TECHNOLOGY The following comes directly from the Brocock design team: ‘One of the challenges of designing the new Brocock Compatto was to meet some of the many specification targets laid down by the sales team. For example, specifications called for the rifle to be short and compact, but capable of high power for export, which also translates to a high shot count for the 12 ft.lbs. versions – something UK shooters are increasingly looking for. In an airgun, where the power comes from the gun rather than the cartridge in a firearm, these requirements tend to work against each other. The designers had considerable experience with the Harper Slingshot system and the group held a patent which was for the first time available to Brocock. The problem was that the existing Slingshot system was developed for the Daystate Huntsman and Air Ranger models back in 2008 and the space available inside the Brocock action was considerably less.

The Slingshot hammer system works by having a central core inside the hammer which is pulled backwards with a small spring. When the hammer is at rest, the hammer cage sites against a stop, but the central core does not touch the firing valve. When the rifle is cocked and fired however, at the end of its travel the cage stops abruptly, and the central core will now fly forward under its own inertia to strike the main firing valve. The strike will open the valve, but not continue to hold it open due to the Slingshot’s return spring. This system eliminates the ‘valve-bounce’ and ca add up to 40% to the shot count.

The problem the design team had was that the core needed a certain mass to work and there wasn’t enough space available. So the team went back to Steve Harper the original inventor of the Slingshot, to ask for help. The new system Steve came up with again proved him to be one of the great airgun designers, and he was able to come up with refined system that allowed the Brocock Compatto to produce energy up to 30 ft.lbs., when the original hammer system used in rifles could only go to 25. Additionally, the shot count was increased, although at this stage we’re not prepared to say by how much!’"
 
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So I tried setting the inner hammer about 1/16" off the valve pin - test speed was 386 to 457 fps with JSB 13.4. The hammer screw protruded about 0.30mm from the inner face of the slingshot portion of the hammer. I suspect that the inner slingshot hammer, as some have mentioned above, does not have enough mass to open the valve sufficiently for the SSH to work properly. One question is whether the black hammer weighs more than the silver one. I have the silver one in my Bantam.

Adjusting the hammer screw out about 1 turn got me 566 fps, 2 turns 727 fps, 2.5 turns 835 fps - then I tweaked back and forth a bit and wound up about 2.4 turns out. The velocity started at 812 fps at 190 bar, continuously dropping to 705 fps at 65 shots, 150 bar (both measurements from the gun pressure gauge, which is close to my tank gauge). I skipped and only loaded a pellet every 5th shot to save pellets. That run equates to 0.92 fpe/ci.

I adjusted the hammer screw out again to 3.5 turns: 808 fps at 150 bar down to 717 fps at 115 bar 35 shots - 0.59 fpe/ci. It is looking like the further the hammer screw is out, the poorer the efficiency.

At this point my valve seat is now leaking, so I need to look at it and see what the problem is. Ah the joy's of buying used....
 
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Here is the answer I got from Brocock tech support about hammer adjustment.

There is no optimum striker distance as every rifle is slightly different with adjustment to allow for mechanical characteristics. The only rule is to ensure not to over extend as power will not increase, but bounce may have adverse effect on air usage. Effectively, once hammer adjustment makes little or no difference to max power, rifle has reached its limit. Parts are identical worldwide with a like for like rifle. Kind Regards - Simon CockayneTechnical Support Manager
 
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