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-   -   Absurd Halfblock Inconsistencies (http://www.mcarterbrown.com/forums/ask-experts/217826-absurd-halfblock-inconsistencies.html)

greystone 11-05-2012 02:28 PM

Absurd Halfblock Inconsistencies
 
I am having issues with a halfblock I built, trying to get it to shoot consistent velocity. I can get it to shoot consistently around ~320 fps, and consistently around ~240 fps, but no where in between. When it shoots around ~320, I'm getting enough blowback to break all the balls in the feedneck. Using a Jcurt bolt.

I have sweetspotted the regulator, have adjusted it higher and lower. Tried running a stock valve and stock slotted hammer. Tried a heavy slotted hammer, it became more consistent, but started randomly venting the valve every few shots.

Using a freshly rebuild Torpedo that works fine on another fullbody pump cocker. I've swapped valve and spring setups from another working fullbody pump, with no avail. Tried blue valve, green main. Tried stock valve, green main.

Anyone have any setup combinations that work for them? I have a plethora of valves and springs available - have almost every variation of stock WGP valve, in addition to Tornado, Shocktech, DYE, ANS black and others.

The internal combo and regulator was extremely consistent before it was halfblocked.

I've tried underboring the barrel with fresh, matched paint - as low as .676 - still horrible inconsistency.

Driving me nuts. I have 5 other pumps that I have built from scratch, but this one is getting the best of me.

Spider! 11-05-2012 02:44 PM

Checked your bolt fit? Fat o-rings can help get the gas to the front, but the front needs to have a decent fit in the breech as well. How far the bolt goes into the breech with the closeness of fit to the bore will affect the blow by.

Put two balls in the open feed neck, shoot one and see how high the second one goes up. Then cock the marker and just put one in the feed neck and dry fire. If the dry fire is consistently lower in blow back, you're getting significant leakage from the front. Tighter barrel fit on the paint makes that worse. Otherwise you're probably getting leakage from the bolt o-rings.

If you are using air, I wouldn't worry too much about sweet-spotting. For that same matter, you might make sure your reg doesn't have a built up ring of wear or old lube in the one area inside that you are trying to run it in. That's some annoying stuff there.

greystone 11-05-2012 02:54 PM

It's a jcurt bolt, no o-rings. The bolt fits tight. Tight enough that I had to spend an evening pumping the bolt in the body to wear it enough to be a smoother pump stroke. Tight enough that if I leave the marker in the Arizona sun, the bolt swells enough that it binds the pump stroke...

I have two freshly rebuilt Torpedo's and both of them work the same on this marker, so I've confident it's not a regulator issue.

Tighter fitting paint/barrel match definitely made the blowback worse when she's shooting up in the 320 range.

wd3d 11-05-2012 02:57 PM

Just an idea here, have you checked the alignment of the air hole between body and bolt?

Spider! 11-05-2012 03:25 PM

Make sure it goes into the breech enough.

Taking wd3d's comment also, maybe your pump rod is a little too long, leaving the bolt back too far, depending on what limits your stroke.

Grendel 11-05-2012 03:33 PM

Just half-locked not midgeted too? Because if you midgeted, that would change your completed spring setup.

maggot 11-05-2012 06:02 PM

If this is a semi, your timing is too tight. It is recocking almost immediately after each shot, causing air to blow into the feedneck.

If this is a pump, you aren't holding the pump handle forward when you shoot (you should be doing this).

/thread ;)

greystone 11-05-2012 09:34 PM

I always hold the pump handle. :)

The sled, pump rod and bolt were made by jcurt, when the gun was halfblocked.

maggot 11-05-2012 09:51 PM

hmmm..
 
Quote:

Originally Posted by greystone (Post 2525816)
I always hold the pump handle. :)

Well, that changes things. The root of the problem is air escaping to the feed (you said it was even breaking balls in there). Is there a gap b/n the bolt and body when you look down the feed (w/ the bolt forward)? It is sounding like your half block job may have been the wrong length.

raptor69r 11-05-2012 09:51 PM

Just trowing this out there, it the lower tube clean of metal shavings and foreign materials? If not that could cause drag on the hammer, also check your rear grip screw, if too long it kinks the main spring or rub the hammer.

Hope it helps


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