Factory Breech Hole Size on QB78S in 177 ?

[wll2506]

I decided to test the QB78s with JSB Monsters now that the seal is working well.. Sighted in at 17.5 yards, was looking to get me a bit more down range energy to see if the Co2 could push that heavy 13.43gr pellet out at a good clip.

I was hoping on the high note to get 605 fps ----- and that is what I got for 23 shots :-) My first shot was 587 and my last was 582. My house was 72 ° most of the shots were above 596 going up to ~610+fps --- I'm all smiles !

Accuracy was very, very good giving me 25 shots, I took the first shot and the last shot out of the equation. I'm now getting over 7fpe at 45 yards and about 7.8fpe at 35 yards --- that is great. I looked around for a BC on this pellet and some were all over the place, these are old style Monsters and I set the BC at .026 (which is probably a bit low) If someone knows the BC of this pellet in 177 please post !

This pellet hit about .5 to the right and about .65 higher than the CPUM pellets that I was using that were averaging 635fps !

Yes, I'm ecstatic on the performance of this gun since the seal was changed !!!

I'll be waiting breathlessly for your response guys ;- )

wll

[wll2506]

What is the hole size of the air hole in the bolt, the hole that is in the bottom side of the bolt, I did not look at it, but for 177 cal guns should it be ~ .125 ? If mine is smaller how much velocity do you think I'll gain if it matches the air port in the barrel ?

wll

[wll2506]

I had an extra bolt mechanism so i made the air hole 3/32 in diameter and took my QB78s apart, the hole on the gun as it came from the factory now is about .100 ;- ) so enlarging it was not needed, I did replace the three barrel "O" rings and put her back together, I left the ice maker tubing in place and I'm hoping I don't have to change that again.

I went out shooting yesterday and my velocity was slow, but after a while the barrel seemed to loosen up, and move from side to side a bit and the velocity went up a lot ------ ~625 fps with the 13.43 gr old style Monsters, giving me over 11.5fpe with these 177 cal pellets. When I put the gun back together today I made sure the the barrel was perfectly aligned and tightened down the barrel grub screw just enough to stop the barrel from moving.

I will test the gun again today after work to see how she does. I'm hoping the velocity is good and above the ~605fps area that I had a few days ago, I would love it to be in the 620fps area with the Monsters I'm using now. The "NEW" monsters I ordered will be coming shortly and the BC on them is much better than the older style, I just hope they shoot good in my gun, but I'm sure they will.

This week end my secretary bought me a few targets to shoot at, they will get blasted this weekend, pics below:



wll

[Ribbonstone]

Ignoring the little stuff in the 1,000ths….basically .11” in the hollow section, .11” for the side hole leading into the hollow section.  Definitely the smallest holes the gas has to run though.  Has the look of being polished up, but not purposefully enlarged.


Never did change that. Running HPA, but only 850psi output.  Wanted 200 shots at 12 foot pounds….worked without doing anything else to the bolt probe, so I let that choke point alone.