Air Gun Home :: View topic - QB79 Ninja

rsterne · Moderator

I tested the efficiency at various hammer spring preloads today, using the 15.9 gr. JSB Exact Jumbo pellets which are very accurate in this gun.... The new slimmer Delrin valve has picked up about 2 FPE, so it shoots the 15.9 gr. pellets at nearly the same velocity as it did the 14.3 gr.... I had to back the preload out several turns relative to the stock poppet because the harder Delrin is much easier to knock open than the soft seal on the stock valve.... Here are the results....

The efficiency seems slightly higher than before, and the plateau starts at 6 turns out instead of 3.5 turns.... Based on the results, I tried a short string at 6.5 turns out, and the velocity was only about 10 fps below the plateau and the efficiency was about 1.10 FPE/CI.... I think that's pretty good for a QB79 shooting at 25 FPE, and it's likely where I will end up tuning it.... That should give me over 70 shots per fill....

Bob

rsterne · Moderator

I finished and installed the stock today....

It's a Richard's Microfit Wildcat Thumbhome in Select Grade Black Walnut with Ebony Grip Cap.... beautiful figuring in the grain....

I had to shorten the forearm and inlet it for the tank block and regulator.... The finish is 7 coats of Watco Danish Oil, each coat wet sanded, ending with a white Scothbrite pad of 1200 grit equivalent and then a coat of Dark Minwax.... The scope is a Leapers 3-9 x 32 AO with Sunshade.... The gun weighs 8 lb. 7 oz. as shown.... The Air Stripper is from a Hatsan PCP, I threaded the barrel 1/2"-20 NF to accept it....

It's no longer a $200 PCP, however....

Bob

rsterne · Moderator

I made one more slight change today before the final testing.... The off center RVA screw was canting the cocking block slightly, and I noticed a slight scuff mark on the spring spacer.... I made a longer, one piece cocking block, with an integral spring guide and spring spacer.... The new cocking block is 1.25" long, of which 0.25" is a spring spacer that is 0.34" diameter, the other inch being full diameter to eliminate it canting in the tube.... That allowed me to shorten the RVA screw and just tidy things up a bit.... It shouldn't have changed the performance at all, but in fact I picked up about 10 fps, and gained a bit of efficiency as well.... The Preload numbers have changed, as I can't put quite as much preload on the spring at maximum with the shorter spacer.... Here is a new graph of Velocity and Efficiency vs. Preload using the JSB 15.9 gr. Exact Jumbos pellets....

As per my usual practice, I selected a preload on the knee of the curve, just below the plateau, in this case 4.5 turns out.... I refilled the tank to 3000 psi and shot a string with the following results....

I got 80 shots between 844 and 856 fps, an ES of just 12 fps (1.4%).... and was able to shoot down to 1100 psi before the velocity dropped below 840.... The average was 849 fps (25.5 FPE) at an efficiency of 1.21 FPE/CI.... During shooting the string, I shot 5 shot groups and sighted in the scope in my 20ft. indoor range.... By the time I was done, it was punching group after group in the same place on the target that were only 1/4" across, barely bigger than the pellet.... I can't wait to try this rifle out at longer ranges.... This is just about the perfect velocity to shoot the 15.9 gr. Exact at, putting it right in the middle of it's sweet spot for the Ballistics Coefficient....

This has been an exciting project, and I'm extremely pleased with the final outcome....

Bob

rsterne · Moderator

I finally had a chance to do some testing outdoors today, so I tested all the pellets scoring "green" in the last tests indoors that I expected to have a decent BC at 25 yards.... I measured the average velocity at the muzzle and at 25 yards, for 5 shots with each pellet, and then used ChairGun3 to calculate the Ballistics Coefficient.... Here are the results....

The groups were sorted as usual, with any group you could cover with a Dime scoring green, any group you couldn't cover with a Quarter scoring red, and in between no colour code.... I was surprised that all the JSBs were the only pellets that produced sub-dime sized groups.... and the best of those was the 15.9 gr. Exact Jumbo.... When you look at the JSB Exacts, the heavier the pellet the better the BC, as expected, and this results in a lot more FPE at 25 yards.... However, with the combinaton of good velocity and high BC, plus superb accuracy, I chose the 15.9 gr. as the pellet to use.... After sighting the rifle in 1/4" high at 25 yards, here is a typical 5-shot group....

The small squares are 0.1", and the black squares are 0.4", so the C-T-C size of that group is less than 0.25".... I can get 70 shots per fill at 25 FPE, and basically it just keeps dumping them in a ragged hole at 25 yards.... I think we can call this project an unqualified success, and this rifle has earned a place on my "wall of keepers"....

Bob

oddtodd · Silver Status Member

Beautiful job as usual Bob. I am going to copy your project except for the stock myself.

rsterne · Moderator

I would suggest that instead of "cock-on-open" you simply trim the hammer spring a bit at a time until you notice the velocity JUST starting to drop.... It won't take much, a total reduction of maybe 1/8" on the LOA?....

Bob

jlwilliams · Veteran Member

Great work as always.

Where did you get the tank & regulator?

rsterne · Moderator

I had a contact in the US, but any Ninja dealer should either have them or be able to get them....

Bob

1970rider · New Member

Bob,
I'm not a machinest like yourself, but I would like to copy your project as closely as possible given my limitations. Are you familiar enough with the available aftermarket valves to recommend one that comes close to your maxi-valve (assuming there is one)?

jlwilliams · Veteran Member

Archer is now offering a repeater kit for the QB79.

rsterne · Moderator

The Archer slotted upgrade valve is the closest thing, and should work fine....

Bob

AirGunEric · Site Admin

rsterne · Moderator

The output side of a QB valve has very generous porting in stock form.... The biggest problem is the transfer port seal, and you can substitute a Crosman one, it fits perfectly.... I do understand the Crosman ones come in 3 thicknesses (although I've never found that), so the thinner ones might not work, but the ones I have used certainly work great!.... Drilling out the barrel port to 0.161" (for .22 cal) and either drilling the nose of the flow-through bolt to that size, or cutting it off and making an extended probe style, complete the Stage 1 mods, which delivered over 17 FPE at 1200 psi with NO valve mods....

Using a Delrin poppet that is a bit smaller OD than the stock one might eliminate the need to bore out the valve to increase the clearance between the poppet and the valve ID (and in fact it might not help much).... I never tried a valve that was stock on the outlet side but with the inlet opened up.... The modifications to the back of my "Stage 2" valve were to bore the ID to 0.50" and drill the throat to 0.219", plus Dremel off the bottom corner of the exhaust port to smooth the corner a bit.... I didn't enlarge the exhaust port, it's plenty big enough for any .22 cal as is....

Archer's slotted valve may deliver MOST of the performance of my Stage 2 valve if used in a QB78.... I've never had one, so I don't know for sure.... Most of the mods to my Stage 2 valve were to increase the plenum size in a QB79....

Bob

rsterne · Moderator

Additional information, March 2015....

Recently one of our GTA members, Ribbonstone, noticed a slight bulging of the end of the tube on his QB79, in the thin area adjacent to the screw holes for securing the tank block.... The gun was operated for a time at 1250 psi.... In response, I have done an analysis of the tank block attachment, which is detailed in this thread....

http://www.gatewaytoairguns.org/GTA/index.php?topic=105712.

If you are building a QB79 HPA conversion of any kind, you should read that thread.... If you are using more pressure than the 1100 psi factory setting for a Ninja SHP regulator, I am now recommending that you pin the tank block with two additional 4mm or 8-32 screws, or a 5/32" shear pin, between the O-rings.... If you want to be more conservative, you might want to pin the block when using the SHP regulator.... or even with a standard 850 psi Paintball regulator.... Complete information is in that thread....

Bob