Transfer Port Size vs. Consistency

[n2omike]

I'm building my .22 Marauder with a Hill valve, WAR SSG and Gaska Hammer Forged barrel.  (when I get a chance to get started on it)  lol  Single parent, kids sports, work full time, etc.  Yea, some of you have been there.   

Anyway, my question is about the Transfer Port.  Hill offers some really nice metal 0.144" and 0.160" transfer ports. 

The barrel seems matched in size pretty well to the 0.160" unit...  but most references I see to .22's, is with the smaller Transfer Port.

My question...  If I'm not looking for that very last bit of velocity, will the smaller Transfer Port do a better job of evening out the velocities shot string?  Would the smaller port be a significant restriction on the gun? 

Thanks!

[rsterne]

It depends on the power you are after in .22 cal.... If you want more FPE, use the 0.160" port and match the barrel and valve ports to it.... If you want less power and more shots, use the smaller one....

Bob

[Dairyboy]

I use a .113 TP from hill. I actually have a hill valve, war SSG and the hammer forged barrel  I get close to 50 shots at 28-29 fpe. Like said depends how much more power you want. I would say .144 would be biggest you want on the .22 for better shot count.

[n2omike]

Well, in a little more detail...   Let's say I want 920 fps...

I can use a larger transfer port and less hammer preload, or a smaller transfer port and more preload.

I'm all for a lighter cocking force, but am more concerned with a more consistent, less extreme spread of velocities.

Will the smaller transfer port do a better job of leveling out the velocity spread?

The larger port is tempting, as it might provide less cocking force, and provide a higher ceiling for velocity, but will it come at the expense of extreme spread?  I'm not looking to shoot super heavy pellets or slugs.

Thanks!

[MichaelM]

personally... I generally tune for the smallest transferport needed to make the power I am looking to get.... tends to give the flattest most efficient shot strings that way.... the great thing with the mrod is you can find a middle ground size then fine tune it with the port screw...

[rsterne]

The FT guys usually restrict the transfer port and lean on the hammer strike to get the flattest, longest shot string they can....

Bob

[Dairyboy]

I can get higher fps with the .113 just like to keep it where it's at with 18.13gr jsbs. Talk to Hill he pointed me in the right direction. What I've noticed is the smaller the transfer port the more shots and a higher fill pressure and a much flatter string than the .144 I have. The larger you get more power at less pressure meaning less shots as well. I've experimented quite a bit and for a .22 im not even sure I'd go above the stock size TP unless your looking for above 35fpe. Unless you plan on regulating then I believe .160 is best per Motorheads advice. The cocking is a little stiffer than stock but just the nature of the SSG.

[rsterne]

It is interesting that people are tuning their SSGs to end up with a higher cocking effort than stock.... This may come from trying to fit everything into the stock "space", as the concept, as originally proposed, is to use a longer, weaker spring to provide the necessary preload while REDUCING the maximum cocking effort, or at least not increasing it.... I have accomplished that on all my SSG conversions.... but in some cases it requires extending the SSG, or the rear plug that contains it, further back behind the original rear of the action.... More pleasant cocking is one of the driving forces behind the development of the SSG, as opposed to just using a short, stiff spring.... which in order to achieve the necessary gap, has to end up being harder to cock by nature....

Just a comment, not trying to drag this thread off course....

Bob

[n2omike]

The FT guys usually restrict the transfer port and lean on the hammer strike to get the flattest, longest shot string they can....

Bob

Thanks for this insight, Bob!  I was torn between 0.144" and 0.160" for the .22...  now it looks like a toss between the 0.144" and the 0.113"!  LOL 

I'm also installing a WAR valve in the .25 Mrod...  and was getting frustrated trying to find a truly reliable 3/16" (0.1875") transfer port...  and am now thinking the metal 0.160" unit from Hill that still uses the factory seals might be the right choice...  as I don't plan on shooting either slugs, or the super heavy pellets.  I don't see going much over 30 grains.  I'm just looking for more velocity, and longer range.  Thoughts?

It is interesting that people are tuning their SSGs to end up with a higher cocking effort than stock.... This may come from trying to fit everything into the stock "space", as the concept, as originally proposed, is to use a longer, weaker spring to provide the necessary preload while REDUCING the maximum cocking effort, or at least not increasing it.... I have accomplished that on all my SSG conversions.... but in some cases it requires extending the SSG, or the rear plug that contains it, further back behind the original rear of the action.... More pleasant cocking is one of the driving forces behind the development of the SSG, as opposed to just using a short, stiff spring.... which in order to achieve the necessary gap, has to end up being harder to cock by nature....

Just a comment, not trying to drag this thread off course....

Bob

I went and purchased the WAR SSG.  I remember the pictures of the original SSG units, and they all had the mechanism sticking out the back of the end cap.  Personally, I believe I'm with you!  I'd definitely rather have a smoother, easier cocking force over the vanity of not having something protrude through the end cap...  as I read the gun can get really hard to cock using the SSG when the spring is turned up very far. 

Thanks Bob!

[Dairyboy]

My apologies if i caused confusion. My WAR SSG is NOT hard to cock whatsoever. What I mean by a little stiffer is I had a 12.5lb spring with zero turns on preload to compare to its maybe slightly stiffer than that and it's only the initial force. It's way smoother to cock though like butter and I wouldn't change a thing about it.

[Tweeter]

The power adjuster on my S400 restricts the transfer port to slow the speed down and the more I turn it down, the more the ES shrinks.  I would definitely try the smaller size first.

[Gerard]

The FT guys usually restrict the transfer port and lean on the hammer strike to get the flattest, longest shot string they can....

Just have to thank you for this tip Bob, along with all the other good stuff you've shared. I've been going to Mission for HFT sessions for about a year, and somehow hadn't heard this particular combination being discussed. In fact there's very little discussion of tuning tips. Mostly when people huddle to talk about airguns it's about new gear, whatever amazingly shiny new gun or scope someone's picked up. I've been struggling to get my QB78D shooting well at Mission in anticipation of EVENTUALLY getting my 13ci bottle connected to it... just a litany of disasters trying to get that going and still no light at the end of the tunnel after 8 months... But anyway, it kept shooting nice strings, if brief at 8 shots, while at home. Then I'd get out to Mission, in March and now last Sunday, and the slightly colder air was stopping it up somehow (grease thickening on the mainspring, differential metal shrinkage, who knows?) so that it'd only start giving me my normal velocity of ~690fps with 18.13gr JSB at about 800psi. Which is silly, as I'd tuned it for starting pressure of 1,200psi. Then I'd get 2 or 3 shots and velocity would fall off again, so going from about 500fps up to 700 then back to 500 in about 12 shots.

With your above advice in hand I tore it down yet again today and swapped out the bolt for one with a smaller port, boring out the bottom hole to 1/8" but leaving the forward hole in the probe stock so that's the choke point. Then I made a new SSG with a much stronger spring to drive my lightened hammer more sharply over its approximately 9.5mm stroke. Night and day! Suddenly I'm seeing a 3.3% ES averaging 727fps - so a little hot for FT targets, but very usable for now in practice. When I get the 13ci bottle attached I'll just add one more thin washer to the spring to bring the setpoint up to about 1,000psi, as that's just a few shots down from the peak and gives about 703fps or 19.9fpe, just below the limit. Once it's been used for a while the spring will probably settle in as well scrubbing another fraction off the velocity for a safety margin. I'd be glad to set up a much softer spring with much longer SSG... but I'm depending on a delrin rear plug with bolt through it to hold my breech in place and mount my tube stock, so the SSG has to be completely hidden and rather short. There's a 1.5mm gap between SSG end and hammer hole end which seems to work well. I've removed any grease so there's no risk of that thickening either. From 3,000psi down to 1,000psi should get me a bushel of shots per fill, and this stronger spring (yeah, harder to cock, but that's alright) with a short stroke is reasonably efficient at about 1.45ci/fpe according to Sikes' calculator.

So thanks again. Wouldn't likely have thought of choking the air in combination with a stronger spring if you hadn't put them together, as I'd been focused on trying to find the perfect 'middle road' with each element in the system instead of one extreme balancing the other.

[rsterne]

I was, of course, referring to an UNREGULATED PCP.... that style of tuning is not really intended for an regulated PCP, as you are not dealing with a bell curve that you have to flatten.... Tuning a regulated PCP involves tuning for (usually) the knee of the curve, ie a point just below the maximum velocity you can attain with the regulator setpoint pressure you are running.... In the "strangled TP . stronger hammer strike" tune for an unregulated gun, you usually don't have to use a stiffer spring, just increase the preload a bit.... Of course if you are using an SSG, you have to adapt your tuning to suit that as well....

Bob