N/A Question for those that tune/machine their rifles.

[Tricky-Dicky]

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.

 

[Motorhead]

Know nothing of the rifle .... but at such a low reg setting and going down in caliber something perhaps over looked is Hammer weight & its stroke ?
Listening to the muzzle report is it a crisp SNAP ... or, a long breathy BOOM ? or, even a rapid succession of short bursts being hammer bounce ?

Excessive valve dwell is not only wasting air, but can have ill effects on the consistency & accuracy if present.

 

[Tricky-Dicky]

Know nothing of the rifle .... but at such a low reg setting and going down in caliber something perhaps over looked is Hammer weight & its stroke ?
Listening to the muzzle report is it a crisp SNAP ... or, a long breathy BOOM ? or, even a rapid succession of short bursts being hammer bounce ?

Excessive valve dwell is not only wasting air, but can have ill effects on the consistency & accuracy if present.

Thanks for the reply, but I made an SSG for it so no hammer bounce and I made the hammer adjustable throw so can tailor to suit.

 

[KatoKevin454]

try different pellets, maybe the size of the pellets are not mating against the barrel and rifling enough causing wide variations in fps, push a pellet through the barrel with a ram rod with the barrel off the gun, see if the pellet is making good contact to the rifling and the barrel, if not try a larger pellet

 

[Tricky-Dicky]

Already use 4.53 and the barrel is previously used in another gun and not pellet fussy, I have no real issues with accuracy just fps variation and I know some will say its fine leave it alone but if I can achieve less than 7 fps variation with a very similar gun its maddening that I can't achieve the same with this one.

 

[sb327]

That low of power requirement would seem to beg for an adjustable transfer port. That way you can run your hammer up close to the peak and not experience as much spread. I’m not sure how that gun is set up as far as transfer port goes so I can’t offer a suggestion there. If the barrel is grub screw type and has a port that has to align, you can misalign and create a choke point but that’s not very repeatable. It would let you know if choking it would help.

Good luck,
Dave

 

[sb327]

Oh, and don’t overlook Motorhead’s suggestion about the hammer just because you have bounce under control. Hammer weight is still a tuning metric and affects the shot cycle. Remember, the low pressure still has to push the hammer back out of the way for the poppet to close (along with valve spring).

Dave

 

[Tricky-Dicky]

It has does have a TP throttle like a lot of guns these days which I have adjusted to no avail, and the hammer had also lost some weight due to machining for the SSG and the adjustable throw so is probably nearly half the original weight, I have enlarged the transfer port from the valve which on these guns is used to throttle down the power for the UK market, and I have also shimmed the valve spring to get quicker closure but had to remove as it needed too much hammer spring to compensate, I have now decided to order a Lane bottle reg and either wind the STD reg adjustment, so the bottle reg is doing the work or use the bottle reg to get the pressure down to say 100 bar and then use the STD reg to get the pressure down to 80bar, I am convinced that it's a reg problem but if it doesn't work I am at a loss.

 

[Motorhead]

Big plenums used with low power seldom have a tight ES as small sips off plenum may NOT get the reg to refresh but every other shot or more. As such each shot is at a slightly different speed see-sawing up & down with the plenum pressure.

 

[Tricky-Dicky]

Big plenums used with low power seldom have a tight ES as small sips off plenum may NOT get the reg to refresh but every other shot or more. As such each shot is at a slightly different speed see-sawing up & down with the plenum pressure.

The plenum is plugged and volume is around 8-12cc, which is what I use on all sub 12 guns, which I mentioned in the first post.

 

[CanMike]

I’ve rebuilt a reximex Daystar / Pretensis. Probably same valve as in yours. The valve may not be suited for such a low reg pressure. I’ve ported and run at 180 BAR and my shot string is flat as flat can be. Maybe plus / minus 2-3 FPS

It could also be the reg. Have you tried increasing it to 120/150 and seeing how
your ES fares. I know you’re probably after a specific sub 12 output but at least you can see if it’s a REG issue.

I have a feeling the whole system is not operating well with such low pressure.

 

[Tricky-Dicky]

The Throne is very similar to the Kral I have, the only difference is the reg, otherwise it's almost the same way I have tuned the Kral and that will give fps below 7 and will often return up to 8 shots of the same fps which is why I am leaning toward the reg.

 

[Racing]

I´d say this is a matter of balance.
To my findings very much can be had by altering that power adjuster. Get or make one that in essence will close the shutter completely, and just open ever so slightly at full.
Point being that such a restriction is a constant and should to greater avail hand what you´re after.

As far as various power adjusters, be aware that the same gun sells in Germany where the British 16 Joules becomes 7..5 Joules - and they get away with it

 

[Tricky-Dicky]

I have now ordered a Lane bottle reg so will see if this is indeed the problem that, I suspect, will update when fitted.

 

[KatoKevin454]

have you tried replacing the breach O-ring, maybe it's worn out.

 

[Tricky-Dicky]

Thanks, but it's new and tested.