Time on my hands today, so ran some tests with Crosmans and QB's.
Tried to keep things as even as possible.
1. Put the rifles and co2 cartridges in the out-building workshop and let them "cook" for a short bit.
2. Set the thermostat on the AC to 86 (F) and let it cycle through a few times.
3. Shot 5 rifles, one shot at a time, in rotation, at a slow pace...with time to jot down notes for each shot...so each had a good long wait between shots.
4. Chronograph was set 5 yards downrange...listed vel, would be corrected MV.
5. ALL rifles had the first two shots "wasted" to be sure the cartridge was well pierced.
6. Target was just a rag stuffed box..no attmept at accuracy testing (have already done that).
7. To help make it an apples-to-apples comparison with the Crosmans, loaded the QB's with one 12gr. cartridges (and a dead one)...can't load the Crosmans with two, so that seemed reasonble.
8. Retest the QB loaded with two cartridges.
Can give a long list of shot-by-shot vel. but that's kind of boring (but I'll do so if someone really wants it). All .22 pellets are 14.3gr. and all 177's are 7.9gr.
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QB 78 (.22). Polished, ported, lubed, a slightly heavier hammer spring,reamed out bolt probe, etc.
ONE cartridge:
Vel. drop off started at round #14..and continued gradually until round #22, where it started falling real fast.
For the 14 shots, vel. averaged 645fps / 13.2FPE+
TWO cartridges:
No real surprise here, it worked just about twice as long as one cartridge did...28shots to the drop off, and at 37shots it started falling like a stone. For the first 28 shots, the average vel was 660fps / 13.8FPE+.(high 672fps - low 639fps)
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AR2078 (177). This one has been polished, ported, lubed, but the hammer spring has been reduced a bit for easy cocking and a bit less speed. Might rethink that after today.
ONE cartridge:
Meaningful vel. drop off started at round #18 and continued at a gradual rate to round #29, where it dropped off fast. For the first 18shots, the vel. averaged 655fps / 7.5 FPE+. (High 670fps - low 641fps)
Two Cartridges:
It's on the list of to-dos..ran out of light.
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Crooked Barn Crosman 2260 (26" barrel .22). Replaced the hammer spring with one slightly stronger, cleaned up any sharp edges, etc. Does NOT have a special valve, is running on a slightly smoothed Crosman valve:
This one got 24 good solid shots before vel. started to drop off, and the vel. drops off much more rapidly than with QB's. Average vel. of 609fps / 11.7FPE+ (High 622fps - low 601fps).
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Crosman 17 / 2250 (26" barrel .177) as issued from the Custom Shop:
This one got 27 good solid shots before starting to drop off...and again, the drop off was quite rapid. Average vel. of 679fps / 8.1FPE+ (high 707fps - low 663fps). that one 707fps shot is way out of line, must be a lite pellet taht escaped inspection...that one shot is 20fps fater than the 2nd highest shot).
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Crosman 2240 (14" barrel .22) as issued from the custom shop:
Little fella got an outstanding 28 solid shots before falling off very rapidly. Average vel. only 519fps / 8.6FPE+ (high 534fps - low 511fps). Don't feel sorry for the low speed, it's a bird killing SOB.
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Conslusions:
1. My QB's are eating too much gas. Know it takes more CO2 to go faster, but using almost 80% more gas to toss a pellet a bit less than 10% faster seems like a pretty poor trade. Going to have to work on balanceing the QB's for the summer heat...any help with ideas here (besides the obvious mucking about with springs) would be appreciated.
2. The large volume of the gas tube seems to be part of the reason QB's vel. drops off much more gradually than the small volume used in Crosmans (basically, the only gas tube on a Crosman is the cartridge itself).
3. There is a vel. gain of about 15fps (on average) for the .22 QB using two live cartridges.
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So guys, here is the question. Lets assume I've done the porting/lubing/etc. right...what i'd want to do would be to knock the QB78 and the AR2078's power DOWN a bit for moe shots per cartridge. Considering I'll not shoot game past 40yards, if i work backwards, find I'd want 500fps at 40yards....which translates to 625fps at the muzzle (for this .22 pellet).
So I want to lose 35fps to see if I pick up a significant number of shots before drop-off. Current plan is to decrease the hammer spring of the QB 78 and to increase the VALVE spring on the AR2078 (and here, I'm thnking of a thicker valve washer rahter than changing the spring).
then again, as the shooting has stopped, it all might just be the harp playing in my head.
