Just what I was afraid of...

[daved]

My gas ram CFX has been shooting some nice tight groups at 10 yards, despite some pretty extreme velocity fluctuations, but I was pretty sure it was going to mess with me once I was finally able to shoot at some longer ranges.  Well, we got all the way up to 40 today, and the sun even came out, so I kicked a path through the snow behind my shop and moved my trap out to 30 yards.  What a disappointment!  My groups looked more like shotgun patterns, the only thing consistent about them was how bad they were.  So now I need some suggestions.

So here's the symptoms.  The last time I chronied it, I'd get a decent string going, the have a 100+ fps drop for a shot or two.  The more I shot, the worse it would get.  Last night, shooting at 10 yards with Superdomes, I had several good groups, 1/4-3/8", when it suddenly started scattering pellets.  The general trend was a drop in POI, as much as an inch or more.  Today, they dropped and scattered after less than 10 shots, and I was getting the same results regardless of pellets.

As I'm sure everyone knows by now :-), this rifle has a gas ram.  I've recently replaced the piston seal, as well as both rotary breech seals.  Nothing seems to be helping, and I'm still way down on velocity and consitentcy.  I should be getting about 900 fps from Superdomes, I'm getting more like 800, with drop outs close to 700.  After replacing the piston seal, I lost about 20 fps, but that wasnt' a big surprise.  I was hoping it would improve the spread, and it didn't.  So I'm out of ideas.  I've inspected the compression tube for burrs, it looked fine, but I ran a hone through it anyway, just in case.  Used 400 wet/dry and didn't get crazy with it.  Anyone have any ideas?  Have I missed something obvious?  I started working on my rotary breech re-design, but broke both tool rests on my lathe, so that's on hold for the moment.  Hope someone can come up with something I've missed or not thought of.

Dave

[DanoInTx]

Find a good deal on an S1K, try the gas-ram in that to see if it's the gas-ram or the CFX...I suspect you still have a leak somewhere in your CFX.  Can't ya try shooting the thing underwater to see where the bubbles come out...hehe!!!!

Dan

[daved]

Actually, I was thinking of doing something along those lined.  My boss has a Shadow, I've used it before.  I was thinking of borrowing it again, and swapping the ram in to see what's what.  May still do that, unless I can come up with a way to try your water test :-)!  Later.

Dave

[shadow]

I got some scuba fins and a mask but you'll have to hold your breath, no snorkle hehe. Hope you get that sorted out though, don't want to see that turn into PROJECT MADNESS. Good luck, Ed

[vinceb]

Is is subject to significant velocity variation as it warms up or cools down?

[DanoInTx]

Sounds like a plan.  I dunno why, but even though I have no experience with the breach on the CFX I am very skeptical of its sealing ability.  I have a friend who is an engineer that I went to Germany with twice last year.  We spent alot of time driving from place to place over there, sometimes 5-6 hour roadtrips, and most of what we talked about was airguns, or airgun valves specifically(CO2 and PCP/HPA stuffs).  We were looking at the huge hydraulic valve used to run the catapult that we use for crash testing.  This valve releases a HUGE amount of hydraulic pressure in a very short amount of time...we were throwing about 700pounds at 16+ G's without the system even thinking twice about it...I think it is actually capable of some ungoddly number of 2 tons at 70 G's....alot of energy.  Anyways, long story shortenned..that system uses a valve that slides, kinda like the breach on a CFX, two miss-aligned holes, all of the sudden are aligned and hydraulic fluid flows.  So you think,"Well how do they keep that thing sealed?"...well, short answer is that they don't, it leaks, just like the CFX breach, the difference being is that the leaks are captured ad redirected back into the system....you have no capture and redirect thing going on with a CFX, just pressure loss.  So in this case they understand that a sliding seal is poor at best, but they overcome that poor seal by accepting that it's poor and seal it from outside the valve.  Now the breach on the CFX isn't under pressure when it slides, so that is a plus, but it's still a sliding sealing surface..and that is a no-go.  I think that there are probably alot of good sealing CFX's out there, but I still think that's the weak link in the design, and if anything is machined just one iota out of "perfect" it just doesn't work right.  Would be best if the breach openned more similar to an RWS48/52 or B21/30, where the back slid fore and aft to make a seal rather than sliding up/down.

just my $.02

Dan

[daved]

I've heard that it can be, but I haven't seen any indication that it is.  Let the fire go out in my shop last weekend, gun sat in 40 degrees all weekend.  Picked it up the following Mon. morning, the first two shots were about 1/4" low, the it came right back to zero.  And the last time I chronied it, it was well warmed up.  So even it it is a little sensitive to cold, I don't think that's the problem.  Of course, when we get back into triple digits this summer, maybe it'll be different :-)!  Later.

Dave