Opinions please, this has got me foxed!

[Tricky-Ricky]

I acquired a Reximex Ixia/Throne sub 12 (I'm in UK) as a project so as I have modded Turkish guns before to very good effect I made it an SSG plugged the plenum volume and turned the reg down to 80bar and added a .177 barrel as it was previously .22, now this setup is very accurate, but the fps is not at all consistent with various weights of pellets.

So re stripped and when through the trigger sears as there was a little too much tension on the hammer from the sear, so that sorted I then had a look at the reg and decided that the transfer port of the reg was very small so bored it out a little more and the sealing face was IMO a bad design and not good for sealing so re machined that and put it back together, but the fps is still not consistent and will vary by over 20 fps from one shot to another, so I am at a bit of a loss, although I may just buy a Lane bottle reg and see if that helps but the STD reg has to stay due to the design so it either run it as a second reg with the first taking the bottle pressure down to about 90-100 bar and have the STD reg handle the jump to 80bar or just open the STD reg up so it's just passing through but I maybe barking up the wrong tree completely, so any input would be appreciated.

[JPSAXNC]

Inconsistent velocities are usually caused by some mechanical function that isn't repeating consistently.  A problem with the hammer travel, a problem with the valve stem travel, or the reg isn't consistent. If it's the reg. tighter fitting o-rings on the piston might help.

[JPSAXNC]

PS. If there's a moderator on the gun take it off for testing every thing else. Hope this helps

[Tricky-Ricky]

Thanks for the reply, I am starting to be convinced it's the STD reg as I have been through virtually everything else, so have ordered a bottle reg, trouble is due to the design I have to keep the STD reg as its part of the air transfer from the bottle to the valve so it will entail either winding up the adjustment of the STD reg and use the bottle reg or use the bottle reg as the main reducer and fine tine with the STD reg.

[rsterne]

If you don't have enough hammer strike for the pressure setpoint, you will have great efficiency, but the velocities will tend to be inconsistent because any small variation in hammer strike will have a large effect on the velocity....



The solution is to reduce the regulator setpoint pressure so that your tune will be more like the red or yellow lines (3-5% below the now lower plateau velocity with lower pressure)....

https://www.gatewaytoairguns.org/GTA/index.php?topic=74919.0

Bob

[Tricky-Ricky]

The trouble is that with low power that we have here It's very easy to go over the limit if the hammer strike is increased, I am running around 80bar and with an SSG and can vary both spring tension and hammer travel/free travel and I have set up much like my Kral which is very stable so I doubt its down to hammer strike.

I initially had the same problem so went through the trigger sears because in this particular rifle a sear is in permanent contact with the hammer, so I have reduced the friction there which has increase deficiency, so I have had to reduce the hammer strike to keep within the limits but the problem persists.

[rsterne]

It is likely what I said.... If you increase the hammer strike with the pressure at 80 bar, what is the velocity plateau, or have you ever found it?....

Try reducing the pressure until the velocity drops to the level you are allowed with more hammer strike than you have now.... ie find out where the plateau velocity is.... and tune from there.... This is the basis for all tuning with a regulated PCP.... Read the article linked above to better understand the procedure....

Bob

[rkr]

It's most likely what Bob said. I had this similar problem after I swapped an 8" barrel on my .177 regulated bottle gun for 11" barrel, spread suddenly went from 3 m/s to 7 m/s with JSB heavies while average speed went from 218 m/s to 225 m/s. Upped the speed to 240 m/s and spread is now back to 3 m/s. As I don't want to play with the regulator that was an easy thing to do and gets me to minimum wind drift at 50M. For UK you'd have to drop the regulator pressure.