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Old 10-18-2001, 08:21 AM   #2
Join Date: Oct 2001
Location: Calgary
An Externally Adjustable Trigger Stop for the PMI Piranha

Greetings folks. The moderator has kindly asked me to post the mod right here in the discussion group, so here it is. There are a few images in this post to illustrate the process, so it may take a while to load. Apologies to those with slower connections.

Pictures will be added to this article, as I get them done.
Future articles in the works:

Adjustable Trigger Undertravel
Adjustable Trigger Travel
Adjustable Trigger Position
Adjustable Droop Sight Mount

Feedback and questions can be sent to me via e-mail.

The PMI Piranha is a great paintball marker. It has good, useable performance out of the box and still leaves room
for the owner to tweak and upgrade, which for some of us is (at least) half the hobby.

This Article is concerned with tightening up the Piranha Trigger, specifically removing the "overtravel" between the point the gun fires and the point that the trigger stops travelling backwards. In the Piranha, there is no solid trigger stop, so the trigger continues travelling back until it touches the frame, or until your brain and fingers catch up with the fact that the marker has discharged. I prefer my trigger to have a hard, tactile stop immediately following the marker going off. For me, this makes multiple shots faster, and easier to hold on target.

After reading several excellent How-To articles on shortening the Piranha's trigger overtravel, I used the described method and added a free pin inside my trigger spring as a stop. After several repeats of "Attach frame, test trigger, detach frame, file" I filed a little too much off the end of the pin. I wasn't, therefore, totally happy with the trigger stop on my Piranha, but I was too lazy to start over. Additionally, the pin rattling in the gun always bothered me. I wondered if there was a way to approach the modification which would not require the repeated removal and re-attachment of the frame to find the right stop length, and a solution that would eliminate the rattle. The following procedure is what I came up with.

Note: It's come to my attention that a recent paintball magazine had a similar modification, but I came up with this one myself (It hardly takes a genius to figure out where to drill a hole to add a trigger stop). Enjoy.


You will certainly void any warranties you have on your gun with this modification. Making a mistake will almost certainly mess up your trigger, and may ruin your frame completely. You are going to be drilling holes in un-forgiving locations, so remember to measure twice and cut once. It's easier to avoid mistakes than fix them. Finally, I'm not accepting any responsibility for what you do to your marker, this is just information and I make no guarantees concerning your results.

When finished, there will be the (recessed) head of an adjustment screw visible just in front of your trigger. Removing overtravel is accomplished by turning the adjustment screw in until the cocked Piranha will not fire, then backing the screw out slowly with the trigger pulled until the marker fires. An extra quarter turn or two past this point will assure reliable firing with every trigger pull.

The priniciple advantage of this overtravel modification over the loose, internal pin-style of modification is that any later mods you make (for example, grinding the sear) which change the release point of the gun can be accounted for in the over-travel stop without removing the frame from the gun.

The principle disadvantage of this modification is that it is much less forgiving of a lack of precision during installation. If this mod is done sloppily, the adjustment screw can bind the trigger return spring and reduce the reliability of your trigger response.

You'll need an adjustment screw that will fit inside your trigger spring. I used a 8-32 X 3/8" Allen stainless steel set screw. The screw has to be long enough to act as a trigger stop and pass through the frame, without being so long that it protrudes in front of the trigger. An 8-32 adjustment screw fills up about 80% of the center volume of the spring, so it's about the largest diameter screw you can use. Use something thinner if you can find it--it will give you more leeway with your measurements.

You also need a drill-bit and tap you can use to produce a threaded hole in the frame that will admit the adjustment screw.

The hole is bored straight down along the (vertical) axis of the trigger spring, through the frame, creating a hole just in front of the trigger, inside the trigger guard.

This may seem obvious, but I'll say it anyway for the benefit of those who have never tapped a threaded hole before: the drill bit should be the same diameter as the central part of your screw (the part of the screw that would be left if you removed the threads). The tap will cut the threads into the sides of the drilled hole, enlarging it enough to admit the adjustment screw.

When drilling this hole, I strongly suggest you mount the frame in a drilling vise and use a drill press.

As with the other trigger modifications, remove the trigger pin and trigger spring from the gun. This can be accomplished with a thin rod (a nail with the point filed flat works nicely) and a hammer, tapping on the pin from the side of the frame with the safety button on it. It's not necessary to remove the trigger entirely from the frame, provided you can get it into a position where the drill bit will not damage it.

You should remove the grip panels from the frame to provide flat surfaces for the vise to clamp and hold. I also recommend removing the sear-release lever and the safety lever, as these parts are held in by the grips and can slip out during drilling, and always seem to fall into inaccessible cracks in your floor. Be careful not to lose the tiny ball-detente and spring for the safety lever (They fly quite far if you flip the safety with the grips off).

If you ignore my advice and use a hand drill, you should be aware of two problems:

The first problem a hand drill might cause is that the bit will wander some distance across the frame before it begins to cut, which may cause your hole to be off-center. This might cause the trigger return-spring to bind up on the screw's threads, preventing your trigger from re-cocking for the next shot. You can use a punch or awl to start the hole and minimise this problem, but a drill-press will not wander at all.

The second potential problem with using a hand drill is that the hole has to be parallel to (and, ideally, congruent with) the axis of the trigger return spring. Boring a non-vertical hole through the frame will result in the trigger-spring binding with the screw threads at certain adjustment ranges. This is bad, and nearly impossible to fix without filling in the hole with epoxy and re-boring. Trust me, that's more work than you need.

You can minimize these problems if you use a smaller-diameter screw, but why take the risk? Use a drill press and be sure. I'm sure someone in your neighborhood has one you can borrow. Who knows, the project might get them interested in paintball, and more players are always welcome.

Once the hole is drilled, tapping is fairly straightforward. Follow the guidelines provided with your tap.

Clean out any plastic scraps and dust that are left in the trigger action, re-assemble your gun and insert the adjustment screw into the hole in the bottom of the frame. It should go without saying that the screw mustn't be cross-threaded in the hole. (See my paragraphs on hand-drilling, above)

Tighten the screw until it prevents your gun from firing, then hold the trigger down and back the screw out until the gun fires. Back the adjustment screw out another quarter-turn or so to account for shot-to-shot variation in the release point of the sear and go play. If you find your gun doesn't fire every time you pull the trigger, you need to back the screw out a little more.

I've not had my adjustment screw change it's settings during play but if your screw is loose and this is a problem for you, wrapping the last few threads in teflon tape should tighten it up enough to hold its setting. Try to keep any teflon off the threads that are exposed, inside the trigger spring, as the tape might cause the spring to bind.

An extreme method to lock down the screw might include filling in the exterior hole with a sealing compound--use something that can be removed without applying heat or strong solvents if you don't want it to be permanent (A plastic frame is a delicate thing). If you've modified an aluminum frame, you can use a thread-locking adhesive.

---Stephen C. Wood Sept 7, 2001
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