Velocity Optimization For a Given Pellet

S

Scrench

New member
Today I set a new PR of 248-7X. I'm using a TM1000 with JSB Monsters, first gen. The thing that bothers me is the 7X. Only 8 of the .177 holes were actually 10's before scoring with a gauge.

I feel like I've found the best pellet for this barrel, and I prep my pellets by weighing to 1/10th grain and careful inspection. Right now the gun is shooting 13.4 grains at 707 fps or 17.65 fpe. Would I notice anything by fiddling with the velocity in terms of trying to get the best speed to match the pellet? I think the gun performed a little better when I switched from the 13.5 grain JSBM's I used on my first target (240-6X) to the 13.4's for the PR target, essentially changing the velocity a little with a lighter pellet. If that is so, how in the world do you determine the best weight pellet and the best velocity to match it?

Thank You!
Scrench
 
Scrench:

I am going to assume that you are using a LW Poly original RAW TM-1000 barrel...

707 fps for 13.4 gr. Monsters is in my opinion too low for a Poly barrel...Monster pellets like to be pushed hard at around 810-815 fps. and at these velocities you would be just under 20 ft./lb.

If you are using a LW 12 groove barrel then you can shoot them at lower velocities, in one of mine I can shoot them very accurately at 745 fps. and down to around 725 +- a few feet...Nevertheless, this barrel tends to be more forgiving at higher velocities..Crank your velocity to 810-815 fps.


Regards,

AZ
 
While 12 groove SOMETIMES turn out good shooters for 13.4's, it is usually not the norm...12 groove barrels seem to like and perform better with 10.3's , 8.4's and even 7.9's and Polygonal seem to shoot better with Monsters, and while some shoot well with 10.3's, very few ones shoot precise with 8.4's...This is why RAW among many other manufacturers ship their guns with 12 groove for LV and Polygonals for HV.

There is nothing written, you need to test your particular barrel...So if I may make a suggestion:
Start shooting those 13.4's at 810-815 fps and maybe come down until you get good precision...If the end velocity at this point is too low and you find that wind deviates much your shots because of said low velocity, then switch to 10.3's and shoot them at max power or close to 20 ft./lb...

On a windy day you will be much better off shooting a 10.3 at 930 ft./sec. than 13.4's at 730

I haven't seen a 12 groove that doesn't shoot well with 10.3's or 8.4's at those max or close to max velocities...

Regards,

AZ
 
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Hmmmmm...

Now you have me wondering. I was shooting AA Diabolo Fields 10.3, and found that the 4.8 or 4.9 head sizes performed the best, but the 13.4's at 707 fps have better accuracy with only weighing them to 1/10th grain. At the current 17.65 fpe, the 10.3's shot at 878 fps. 19.5 fpe would get me 923 fpe. Do you think only an increase of 45 fps would make much difference?

One other thing that might affect this. The gun has always shot decent groups at 25 meters, but when I reach out to 50 yards, the groups are really bad, as much as 2.5-3 inches. But mysteriously it begins to shoot tight again at 75 yards. this is with the 10.3's. I thought some weird wobble might be affecting them on the way to 50. I have not tried the 13.4's yet, but was hoping that maybe their shape or stability might produce tighter groups at 50.
 
"Do you think only an increase of 45 fps would make much difference?

Yes...I've seen 10-15 fps. making a notable difference in many guns...

The fact that your gun came with a 12 groove barrel tells me that maybe that particular gun was originally set for Light Varmint at 12 ft./lb....Those LV guns shoot well 10.3's at around 715 fps. and 8.4's at 785-790 fps...

Do you know what Reg BAR setting you have and if the transfer port has been modified?...The issue you describe that happens at 25 & 50 and corrects itself at 75 seems to be over stabilization of the pellet at certain velocities that can be corrected with a longer/heavier pellet or by lowering the velocity...The other issue that can be causing it is the timing of the gun for that particular barrel not being right...Timing is the relationship between Regulator pressure setting, port size and amount of time that the valve stays open.

I machine my parts and tune all of my guns but I don't do it professionally...If after what we have discussed you can not get it to shoot, your best bet is to contact Ken Hicks in Florida at: https://spaweapons.com/ ...On top of being one of the best shooters in the USA he is an excellent tuner for RAW guns.

Regards,

AZ


 
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I was thinking about tuning it down to 12 fpe and try it with 10.3's again, but first I'm going to try max fps with the 13.4's. My new scale just came in so I'm reweighing everything into groups of .04 grains.

I don't know anything about the reg setting or if the port has been modified.

Thanks for the suggestion about Ken, I'll definitely get in touch with him.
 
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