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PostPosted: Sun Jan 14, 2018 7:55 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8618
Location: Coalmont BC
I finished up the receiver and barrel mods today and made a new hammer.... Here is a photo of the hammers, old and new....

Image

The cocking pin is in the same place relative to the front of the hammer, but the sear notch is forward 1/4".... and the back of the hammer is shortened to save some weight.... The new hammer weighs 43 grams instead of 55 grams.... a reduction of 22%.... The hammer stroke is now 3/4" instead of 1/2".... and using the "cock-on-open" parts shown above and a stock QB hammer spring I have 10 turns of preload adjustment, from 3 turns of preload to 7 turns of gap, using the 10-32 adjusting screw.... It ends up that was perfect....

The barrel port was drilled out to 11/64" (0.172") or 78% of the bore.... and a transfer port made from 1/4" Teflon rod with the same size hole.... I drilled a hole in the loading tray area of the barrel to allow it to be assembled to the receiver and the entire assembly fitted to the gun.... since the flat head screw under the loading tray can be accessed with a Phillips screwdriver through that hole.... That flat head screw now lives (permanently) trapped between the barrel and receiver.... I also installed two 6-32 setscrews in the sides of the receiver that tighten against the back sides of the barrel loading tray.... which really stiffens up the assembly.... The bolt was shortened (flow through portion cut off) and a 3/32" probe fitted that pushes the pellets just ahead of the barrel port.... I shortened the stock to just clear the back of the 88 gr. cylinders.... and assembled the gun for the first time....

Image

I weighs exactly 8 lbs. as shown.... I have done some shooting, and the performance exceeds what I was hoping for.... A report will follow once I have a chance to organize the data.... but the maximum velocities are 1000 fps with 18.1 gr. (40.2 FPE) pellets and 910 fps with 25.4 gr (46.7 FPE).... with both pellets up on the plateau at 1800 psi.... 8) .... The knee of the curve occurs with some gap between the hammer spring and hammer, so I expect to see the benefits of a Short Stiff Spring installation, even though I am using the stock spring (with a gap)....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jan 15, 2018 12:51 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8618
Location: Coalmont BC
As promised, here are the results.... First, the data showing velocity vs preload (or gap) on the hammer spring, at 1800 psi, with the two pellet tested, the 18.1 gr. and 25.4 gr. JSB Exacts....

Image

I was delighted that the new PEEK poppet with the small head and small stem was so easy to open.... Of course the 50% increase in hammer stroke would have helped a lot to make up the difference between CO2 pressures and the 1800 psi I am running on this HPA conversion.... Still, I am only using a stock QB hammer spring, and even with 1/16" of gap (2 turns) between the spring and hammer when uncocked the gun is able to reach maximum velocity with this combination of porting and pressure.... The downslope of the velocity curves are very steep (it doesn't take much of an increase in gap to drop when below 800 fps).... and the knee of the tuning curve is very pronounced.... making adjustment of the gap quite sensitive.... Just 1/2 turn can move you from a perfect tune to starting at the peak, or starting to get valve lock at the beginning of the string, with a big velocity drop.... I played a bit with the adjustments and came up with the following strings, starting from an 1800 psi fill....

Image

The performance is very similar to the Beeman Chief I just built, except for a lot more shots from the larger reservoir (290 cc instead of 135 cc).... The velocities are just slightly lower than the Chief, a result of the slightly lower fill pressure (1800 vs. 2000).... I don't think the larger porting on the Chief actually made any significant difference, and I am wondering if the 3/32" bolt probe, which was shared by both guns (which are both .22 cal), is the limiting factor to the flow.... In theory the area around the probe in the barrel (chamber) is the equivalent of a hole 0.198" in diameter, and on the Chief all the ports are larger than that.... while in this QB conversion the transfer and barrel ports are only 0.172".... It may be that just as we like to see the throat area (after the valve stem is deducted) a bit larger than the porting.... the same thing applies to the chamber and the bolt probe.... I know that using a retracting bolt to completely clear any obstruction from the air path in the barrel helps performance in bullet shooters, where we are after full bore area porting.... It may well be that once you go larger than 80% of bore for the transfer port (which is as large as you can go with a drilled barrel port without possible loading problems).... that to see full benefit you have to retract the bolt out of the airflow.... With a strictly pellet shooter, I could probably get away with a smaller probe (1/16") and still seat the pellet fine.... but with a bullet shooter (which I may do with the Chief) such a small probe would sink into the back of the bullet, because you have to push harder to chamber them.... In any case, a smaller probe would not likely change anything on this build, with the porting less than 80% of the cailber....

The astonishing thing, for me, is the efficiency of this gun.... The total reservoir volume (88 gr. cylinders at 121 cc each plus the QB78 air tube) is about 290 cc or a whisker over.... let's call it 18 CI (295 cc).... From an 1800 psi fill, with strings with a 4% ES, the strings end at 1370-1380 psi, using only about 420-430 psi for the string.... For the 23 shots at an average of 934 fps with the 18.1 gr. JSBs (35.0 FPE) that works out to 1.51 FPE/CI.... and for the 20 shot string with the 25.4 gr. JSBs at an average of 846 fps (40.3 FPE) it works out to 1.55 FPE/CI.... The pressure only drops about 20 psi per shot.... When I reduced the spring gap a turn with the 25.4 gr. pellets, and got that short, declining string (in blue), the efficiency dropped to 1.16 FPE/CI, for 10 shots within a 4% ES averaging 874 fps (43.1 FPE) on 300 psi of air (30 psi per shot).... It just shows how quickly the air usage goes up if you try and get too much power, using 50% more air to get only 7% more FPE....

I am extremely pleased with how this HPA conversion turned out.... The QB has proven once again how versatile it is.... and this one has enough air on board to be quite a powerhouse, despite its modest 1800 psi fill pressure....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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