2240 bolt probe mod for velocity

[cheewee]

sure is a pain changing probes but at least i know my modified t.p and beefed up breech screw are good to go, gotta love a breech screw which uses a 1/16 allen key the original .050" sure would not have done it.

[HappyHunter]

... its tough to get the flow-thru to beat
the extended until you get to .22 and above.....

I realize it pretty much boils down to individual preferance, but even in .22 or .25 I would stick with extended over flow-thru. The following is a post by Rsterne with definitive results on the matter. Yes, it's on a QB, but the principals are the same for all you "power-monger's" (myself included) looking to squeeze every bit of fps you can get:

Well I did a VERY interesting test today.... There is an ongoing argument about which is better, flow-through style bolts or ones with an extended probe that seat the pellet skirt just past the transfer port.... I've been favouring the probe style, but today I got to finally prove it.... I chucked the .22 cal QB bolt in the lathe, and machined off the front of the flow-through section, back to the rear edge of the transfer port hole.... I then drilled a 1/16" hole down about 1/4" past the O-ring groove and fitted in a piece of 1/16" music wire of the appropriate length.... I fitted a sleeve over the wire made from 3/32" OD x 1/16" ID - K & S brass tubing from a hobby shop.... I adjusted the lengths until the probe was 0.100" longer than the front of the original bolt to make sure that any pellet would be seated with the skirt clear of the front of the transfer port.... A couple of drops of crazy glue (plus a good fit) hold the pin and sleeve in place....

I stripped off the .177 cal barrel and bolt, and installed the .22 barrel and the now probed bolt and proceeded with Chrony testing.... I used the 18.1 gr. JSB Exact Heavy pellets which the gun particularly liked.... Here are the before and after numbers....

Number.......... Flow-Through...... Probed
Avg. Velocity......871 fps...............903 fps
Avg. Energy.......30.5 FPE..............32.8 FPE
No. Shots..........40........................37
Total Energy......1220 FPE.............1214 FPE
Efficiency...........0.85 FPE/CI.........0.85 FPE/CI

The probed bolt.... with NO other changes from the flow-through bolt.... increased the velocity by 32 fps and the energy by 2.3 FPE (7.5%).... The total energy per fill remained the same, which reduced the shot count by three shots.... There is no longer any question in my mind that a probed bolt is superior to a flow-through style in terms of performance.... at least in a higher powered PCP.... Just to double check, I also tested the RWS 11.9 gr. Hobbies, and the EunJin 32.1 gr. Pointed pellets.... the Hobbies gained 35 fps (1.9 FPE) and the EJ Points gained 20 fps (2.0 FPE).... so the results are not limited to the JSB pellets....

Besides, this is one of my (purchased) flow-thru bolts after only 200 shots  . I never had this problem with any extended probes I ever made...



Cheers,
Todd

[quickster47 †]

Did you contact GMAC about replacing that probe?

Carl

[arkmaker †]

you could always go with this design. you will not see any boost in fps, but seating the pellet is a charm.

[HappyHunter]

Did you contact GMAC about replacing that probe?

Carl

Not directly. I contacted the Canadian vendor where I purchased it, who in turn contacted GMAC. Apparently it wasn't an isolated incident as both GMAC and vendor have had a few cases where this has happened. Turns out it was a combination of probe being bored just a hair to deep, not leaving enough meat between probe/o-ring groove, and probe diameter being a whisker to big (.217") which made for a (too) snug of a fit when closing the bolt. I guess the fabricator was having an "off day" 

Really wasn't a big deal for me though. Had an extra flow-thru and it's fine. Besides, it gave me a chance to try making an extended probe bolt, and I haven't looked back since! 


Todd

[HappyHunter]

you could always go with this design. you will not see any boost in fps, but seating the pellet is a charm.

I remember following that thread quite closely. Great job!

Seems to me like you get the best of both world's with that design 


Todd

[howie1968]

Thank you all for the great input!! thois type of discussion was exactly what i was looking for once again thanks

[WyoMan]

one more thing howie or three (if you ever use flow-thru)
flow-thru bolts should just barely clear the transfer port in length or there will be too much transfer volume
the port in the flow thru should be checked for alignment with the TP
the ID of the flow-thru can be enlarged (helpful with smaller bore)
Here's a pic of one I modified for .22:

notice how much I cut from the end from the other GMAC bolt

[BDS]

Good info Wyoming... and you indexed the hole on one side only? Nice.. 

[WyoMan]

You be right Brian, had to index the port a little some of it because I openned up the TP a little - thanks 

[KevinP]

the ID of the flow-thru can be enlarged (helpful with smaller bore)

I don't understand this ?

[crossliner]

the ID of the flow-thru can be enlarged (helpful with smaller bore)

I don't understand this ?

Perchance he was referring to them .177 probes/barrels Mr. K..

[WyoMan]

smaller bore but meant smaller caliber...
this keyboard shows what I type instead of what I meant 
the smaller caliber bolts with the same wall thickness as the larger ones have less opening vs bore diameter
so there is more flow restriction unless you make the wall thinner
as the diameter increases you don't have as much of a problem with the larger caliber
Thanks,

[KevinP]

Ahhh  thanks ...   

[sshewins]

I took a stainless extended probe and put it in my cordless drill, and used a file to turn it down some. I felt the tip mushroomed out too much. Cheewee, care to elaborate a little bit on your non .050 breech screw?