Author Topic: QB78 breech bolt end mod.  (Read 4196 times)

Offline wind_watcher

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QB78 breech bolt end mod.
« on: December 01, 2009, 07:45:27 AM »
My new QB78 deluxe arrived some days ago and is now all in pieces. I am going through the steps to make this rifle into the best it can be - thanks to the may posts on this forum. I live in a remote wilderness area of BC Canada, yet linked to the outside via satellite - isn't technology great! I just wanted to add that the creativity of many in this arena is amazing - I did not realize so much beauty could be shaped into a rifle stock. It's inspiring and my gratitude goes out to all of you.

The condition of the new QB78 parts were generally good - although, as often posted, the machined edges were sharp and some burred. The gold anodized aluminum trigger is pressed into a steel piece that was quite rusty and did not move easily on the pin. Somehow I had expected to see oiled and greased surfaces and if there was an oil film, it must have been r e a l thin. My conclusion is that even if I were not mechanically inclined, I would still go through the basic dis-assembly if only to hone, deburr, break all sharp edges, replace the o-rings (almost all of mine were damaged) and lubricate. An excellent graphic set of instructions comes from http://charliedatuna.com and http://airgunartisans.com/flyingdragon/ along with so may who have "been there, done that". It's comforting.

Has anyone ever tried to modify the breech bolt end by grinding a slot instead of drilling the existing gas transfer openings? It seems to me that after a smoothly shaped 90 deg curve is ground out of the probe that there is enough of a bolt end face to seat the pellet into the chamber. The radius of the transition would help direct all the gas flow. I even thought of filling the end of the drill with epoxy so that it does not extend beyond the side port opening (in the wrong direction). Several posts note the weakness near the o-ring seal when drilling, as they tend to break. Can anyone find any objections to a slot - using a Dremel with a small diameter disk? I'm kinda shy at being the first as these parts don't grow on the trees around here. Now if I had a box of them ... I wouldn't even ask :)

Offline Bentong

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Re: QB78 breech bolt end mod.
« Reply #1 on: December 01, 2009, 11:59:12 PM »
On my QB78d I just drilled the probe the size of the transfer port. FPS increase is achieved when the flow form co2 from tube to skirt of pellet is of least resistance. From what I see on doing the slot on the probe, there is still a restriction leading to the slot. I followed CDT tune minus wire probe I just matched the porting and left the strike pin forco2 carts option. Got mine bulked and can use 2 carts by replacing the end cap. Shoots 14.3 CPHP @ high 6's and low 7's depending on temp. I get 30 full shots bulked and 40-45 on 2 carts.