1377 Transfer port - optimum size?

[harvey_s]

Hi all

I recently bought a nice S/H 1377 after getting the idea that I wanted a .177 ultra-carbine.
Soon I got it I found it was a lot lower powered than I expected and a little digging on the net got me the info that it was restricted model to suit the Canadian market & a thread on another forum I frequent came up with a derestriction method...

After plugging the vent hole in the back of the valve, I followed the advice on the thread for drilling the transfer port (2.5mm) and ended up just like the author did at around 4.5 ft/lbs....a tad low for my mini-carbine I thought so...

I upped it to 2.8mm....Result! 5.25 ft/lbs

Now this is where it gets odd...I thought I'd open it out a bit more after all the US standard transfer port is 3.8mm and they are not over the UK limit with these fitted - so out comes the drill again and I thought go easy and try 3.5mm first - Bugger! just over 5 ft/lbs....

Next comes a call to GMAC for a full size US spec transfer port and one of his brass restricted ones which I opened back up to 2.8mm...

Result with US 3.8mm port - 4.7 ft/lbs

Result with 2.8mm port - back up to 5.2 ft/lbs again

The gun has a small o-ring seal that I've added to the top of the transfer port, but is otherwise standard...has anyone else experienced anything similar or experimented in this area?

[breakfastchef]

A transfer port diameter too large will rob a .177 of power. You found an optimum port diameter through experimentation. Lots of work on this has been done by rsterne who frequently posts on the GTA forum. He found that a gun capable of shooting over 800 fps in .177 may benefit from larger transfer ports.

The two easiest ways to increase power from the current level is to add a longer barrel and/or install an adjustable flat top piston and flat top you existing valve.

I never used a top seal on the transfer ports of my Crosman pistols. I tried various times with little to no effect on performance.