Biohazard Transfer Port Mod
Besides the trigger tune...this is the most popular mod.
1- Stock barrel is at the bottom.
2- An early mod for me is the next one up.
Notice the flat spot around the hole. It
is so the breech seal will not get stuck
on the outer edges of the barrel. I used
to actually drill a hole through a piece
of solid plastic rod and then spin it in
a drill while a file was held on it. This
gave a nice angled end to fit into the
barrel. Then cut and sand the other end
flat for the valve. If you were wondering
about that flat spot I turned the bbl around
backwards and rethrouted it cut off the
breech end and recrowned. Way to much
work. I have a 22" bbl gun that shoots
in the low 700's @ 70-72 deg. with this seal.
The tube was not drilled as with later seals so the port hole is not the 3/16ths
as on other guns.
3- The interchangeable
bbl mod., next one up, allows you to loosen a set screw
or two, depending on if you have a barrel band or not, and put on another
length or caliber bbl. Three grooves are cut into the bbl. One as near to the
breech end as possible another in the center of the bbl port. and a third just
beyond the bbl port. The depth and size of the cut depend on the selection of
o-rings in your area. The ring should extend just .003 to .005 above the bbl
for most bbls and breeches to get a good seal. Extra holes can be drilled in
the bbl for higher gas flow. Smaller holes are less likely to damage the pellets
so long as you clean up the burrs on the inside after drilling. Oil the o rings
and just push it in. If you cut the o-rings then your grooves are not deep enough.
Remove the set screws and polish the interior of the aluminum breeches prior to
this mod for best results. Steel breeches are best for this as they do not scratch
up when the bbl is pulled out. I have a gun with 24"bbl shooting in the 720's
@ 70-72deg with this mod. and a 14 inch bbl pistol at about 620fps at the same
temp. A piece of poly tube is used between the valve and the breech only. See pic
should not stick up much past the main tube. To get the extra gas flow and easy
breech seal of this mod without machining you can put a small bit of JB weld in
the breech where the barrel will hit and a small bit at a measured distance on
the barrel where it will be just inside the breech when fully inserted. . Slide
the bbl into the breech and then twist it around once or twice to evenly distribute
the epoxy. No you can't easily remove the bbl anymore but you should not have
any leaks.
4- Next bbl up is countersunk and enlarged one step smaller than that of the
top one, so it's less likely to cause pellet damage when loading. Uses poly,
vinyl or
hard drilled plastic for a breech seal, seals to the bbl. That's 11/64th's and
19/64th's on the bits.
5- Top bbl is what seems would be optimum for the gun as it matches the valve
port ID. The hole is 3/16th's of an inch and is angled for 5/16th's tubing the
tube will need to be drilled for this one as the seal is to big to fit through.
Seems this is a mighty big hole in the bbl to me and the pellet could easily be
damaged. I have never used this size. But use one step down as in #4 or smaller
for the port on some of my guns.
Now for the breech seal that has been spoken of before. As you have seen it can
be made of various materials including metal. It can seal from the valve to the
breech or valve to the bbl as you wish. The hole in your breech can remain the
same and smaller tubing used see pic or it can be drilled out to 5/16th's and
larger tube can be used.
The vinyl tube is your friend if you have leakage problems as it conforms to
imperfect surfaces best. But on the down side you can loose fpe cuz it compresses
and the hole gets smaller as its mashed inside the gun. 1/8th inch is all you need.
The hard plastic stuff is very good but must be shaped and cut to very close
tolerances or you get the leaks.
What's on the 30 fpe DAQ 9mm? The poly stuff shaped at the top slightly with a
razor while installed in the gun then and then sanded to better tolerances by using
an old breech and some fine sandpaper. Recently learned ,from a post on the forum,
that another way to do this is to heat up an old breech and touch the plastic
tubing to it to get the perfect shape. I will try this on my next gun, it sounds like
a good idea. I have old gun tubes, with large holes in them, the inside of the tube
should be the right shape to do the same thing.
Small files are good to remove burrs caused by drilling. Pencil with a piece of
sandpaper taped to it works well for polishing the barrel port or crowning a barrel.
What more can I say? So many ways to do this and get good results, Have fun.