My 74 has a solid tube (unvented) what would happen if I use the vented tube?? I saw a Ultimak rail in the local trader for $70 and it says it is vented. Any problems, ya think??

Nope.

I've wondered the same thing.  My MAK90 has the vent holes in the gas tube, where as the SAR1 that I have, has the holes in the gas block.

If you used the vented tube in conjunction with the SAR1, which has the holes already in the gas block - wouldn't it affect the cycling of the action?  And vice versa on the MAK90, not using a vented gas tube, would it possibly cause too much pressure during the cycling of the action and cause problems down the road?

I'm too chicken to try the MAK90 scenario.  The worse that could happen with the SAR1 scenario is it wouldn't eject and load the next round.

Anyone out there try these out?

DK

If you combine a none ventilated tube with a non ventilated gas block, you'll just end up with more fouling in the action of your gun.  

And as far as I'm concerned a vented tube will not fit a vented gas block without modification because it's slightly too long.