Bucking Suggestions

[Trev140_0]

Ok, I am sure this is standard issue stuff here it goes:

I have a M4.(m16) (version 2)

Was getting readings on the chrono that was all over the place. Spring is M120.

Opened her up. Seal was less than a wet fart after a good night of beers.

Changed the o ring. Ton of compression now. Dry fired it, even sounds like more bark.

Trouble is now its coughing and misfiring bb's. I am fairly sure its the bucking.

When I pull the barrel and put a bb in the bucking the bb nearly falls through. almost NO resistance.

So my assumption is I was shooting around 280 fps before and now going up to the 400 FPS range the bucking is in need of replacement.

Thought in buckings? Or am I off base?

[slink182]

Check your bucking for obvious damage/wear. Also, confirm that your hop is turned up, and that the hopup wheel isn't free floating.

If your hopup is good, then yeah, likely your bucking's gone.

[Trev140_0]

Here is another gut feeling. When I stored this gun last I gave it a good shot of silicone spray.

Does the silicone have a "swelling effect" on the rubber? It was REAL wet with silicone and I am wondering if it swelled it even a bit?

[MaciekA]

Compression checklist:

From left to right, looking at an open gearbox:

1) Cylinder and cylinder head. Make sure the head is sealed nice and tight on the cylinder. If it isn't a super tight fit, grab some Teflon sealant tape and carefully wrap it around the head where the o-ring is, but make sure none of it passes the padded edge of the head. You dont' want any Teflon tape protruding into the interior of your cylinder. Tidy up the edges that overlap the alignment holes and roll back the bit that covers over the area where the tappet plate moves.

2) Inspect your piston's o-ring, make sure it's in good shape. Stretch your o-ring a bit with fire by wrapping it around your cylinder and then applying some heat from a lighter to it for a minute or two. Rotate slowly like a rotisserie chicken. The cylinder will get very hot so hold it with some pliers. Let it sit for a few minutes before you re-lube it and put it back on the piston. Some people let it sit for a bit longer, I like to let it sit over a few minutes. Relube, put back on piston.

3) Now it's time for a cylinder + head + piston compression check. Remove your nozzle. Put your finger tightly over the output of the cylinder head. Now put the piston in the cylinder. You should be gripping the cylinder securely. Now pump the piston inwards at a decent speed. Try to ensure that the piston head's face is more or less aligned flat against the cylinder head. Your piston should stop in its tracks after a few millimeters. If it's still able to move but requires a *lot* of force, that's usually okay and is rarely the source of your problems. If you're unhappy with the seal, consider picking up one of the MASK brand o-rings from airsoftparts.ca. I've had great luck with them.

For the next few tests I recommend removing your main spring, gears, trigger, ARL, etc. Keep it simple..

4) Now you're going to stage the cylinder in the mechbox. Put your airseal nozzle on to the cylinder head and attach the tappet plate. Stage all of this in your mechbox. Put the tappet spring on the tappet plate and let it hold the nozzle in the forward position. Your nozzle is now staged in the position it would be in if it were mating with the bucking. Put your finger over the end of the nozzle to seal it, firmly, but don't let the nozzle move back into the gearbox. Have someone or something like an extra finger hold the tappet plate in place so that it doesn't move backwards. Now carefully pump the piston in just like before. You should get a decent seal. At this point, if you have a sucky seal around the air seal nozzle and cylinder head output, you may be tempted to want to replace them. This is where things can get tricky with fitment (see next few points), but I've had good luck with Modify's cylinder head and airseal nozzle with o-ring (the nozzle actually has a tiny o-ring in it. Some nozzles in guns like G&P already have an o-ring like this).

5) Keep your mechbox on your work table. You're gonna go grab your hopup and barrel. Leave them attached as they were when you removed them from the gun. Now seal your mechbox up, you don't need to use all the screws. just enough to be properly sealed. Put one hand on the mechbox, grab your barrel/hopup with the other, and shove it against the mechbox, like this:




Compare the above two pictures. See how there's a bit of a gap in one of them? That gap forms when I ease up on the shoving. When you press your hopup against your mechbox, you should feel the nozzle actually getting pushed into something soft and squishy. If you feel that, that means your nozzle properly contacts your bucking, ASSUMING that your hopup is pressed tight into your mechbox. This brings me to the next step.

6) In the last step, we ensured your bucking actually made contact with your airseal nozzle. It's important that this actually also happens in your gun. Basically when you look into your ejection port you should see something like this:



See how the hopup is super tight against the mechbox? This rifle shoots 400fps steady. In my experience, many guns that have trouble with seal at the hopup unit somewhere simply do not have enough pressure against the hopup to secure it against the mechbox. Sometimes this is even the case when your hopup unit has a spring. Go ahead and test to see if this is the case in your gun. You want it to be TIGHT. If it isn't tight even with a spring, it's time to resort to creative solutions. If you need more pressure against your hopup, you can always go nuts with Teflon tape and o-rings, like in this picture:



The extra tape around the front of the hopup unit will compress and bunch up. In your first attempt you'll put too much and you won't be able to slide the receivers together. Experiment until you have the right amount. Make sure to shove a mag in there for good measure to make sure BBs are still released by the feeder tube -- you don't want to misalign things.

6B) OPTIONAL - Tappet plate mod: If you find that you STILL can't get your nozzle to seal enough, you can also try removing material from the front of your tappet plate. Look down at your tappet plate when it is in the mechbox and imagine removing material from the front face that hits the front of the mechbox such that it could move forward another 0.5 - 1mm or so. If you think this could buy you some extra seal against the bucking, you can remove material from the front face of your tappet plate. You should have some spare tappet plates on hand in case you mess up, or in case you have a very high RoF setup (40rps+) and expect to shatter tappet plates often.

7) Cylinder-to-barrel compression test. Open up your mechbox for another try at pumping the piston. This time, place your hopup unit right up against the mechbox (remember, *tightly*!). Your nozzle will mate with your bucking. Now place your finger over the end of the inner barrel. Pump that piston and make sure the whole system is sealed.

This should be your reaction:



8) Paper-over-hopup test. This is another way to test if your nozzle makes a correct mating with the bucking. Re-assemble your gun 100% complete. Now turn it upside down so you're looking into the magwell. Place a small square of tissue over the hopup feeder tube. Fire a shot. If the tissue flies up into your face from a blast of air, you have a leak at the nozzle/bucking mating point. If it doesn't move much (or only gets rattled out of position but not flown up), then you're good to go.

9) Time for the bucking lip BB feeding test. Take your hopup and barrel, hold the barrel diagonally so that the end is towards your work desk. Now grab a spare and clean BB. Make sure the feeder tube that mates with the mag is facing up at the ceiling. Now drop a BB into the hopup unit through the nozzle entry hole. Now grab a pin or small screw driver. ***GENTLY*** press on the BB to let it roll into the barrel. Make sure the other end of the barrel isn't sealed or it'll prevent the BB from going down the barrel (you've played with straws at McDonalds before right?). Here's where things can get nasty. If your BB takes a ton of force to push down into the barrel, your bucking's lips are protruding too much into the hopup unit. This is a source of crazy jams, double feeds, and piston shreds. If you are jamming BBs here you can create a seal and the backpressure can cause your piston to seize, at which time your gears will happily tear into the piston. Play with your bucking's insertion and other buckings to find a happy medium between the BB being held in place and going down the barrel. It should take a light touch to push it down.

10) Dental floss mod / Teflon tape mod. Note that this step is not 100% necessary. You should only do it if you want to perfect your seal and you have some spare buckings around (in case you damage a bucking while fiddling around). Sometimes the bucking itself doesn't wrap around the inner barrel that tightly and air escapes between it and the barrel. There are a number of ways to prevent this. You can either wrap a VERY thin (going around twice is usually enough) strap of Teflon sealant tape around the barrel before putting the bucking on, or you can put the bucking on and very very tightly wrap it with dental floss. Here is Maekiii's video on how to do the dental floss mod: Hopup perfect seal mod DIY (floss mod) Airsoft - YouTube The dental floss mod works VERY well. A PDW build I just did is firing 425fps on a M120 spring and is using the dental floss mod as well as the tappet plate mod.

By the way ... I never spray silicone into my hopup, ever. People say to do this all the time, "it's good to do it", etc, but have no good mechanical or experimental argument for doing so except what I suspect to be cargo cult reasons. Spraying silicone into your hopup doesn't solve anything, it just addresses symptoms rather than solving problems. It ensures the problem will come back, and also virtually guarantees terrible accuracy.

I've probably missed a bunch of stuff, but this is basically what I do when testing compression. Some people on this forum will also recommend checking stuff like nozzle alignment, ensuring you have the right length of nozzle, making sure you use good BBs, avoiding super tight tightbore barrels, etc. That's all good advice.

[Trev140_0]

This is all fantastic stuff as usual Maciek! You are always on the ball on this.

I think however the bath in the silicon spray along with letting it sit in the bath has softened and swell the bucking to a useless blob.

I am sure the 70 deg rating is now something way softer now.

Any suggestions on the degree of bucking for a higher fps gun?

[lurkingknight]

what kind of silicone spray was it? if it was from an aerosol spray can, then it had petroleum distillates in it... which don't react well to most types of rubber/silicone.

[Trev140_0]

ya-- the spray shit. Shot it in the hole for the mag feed.

It apparantly is on the KWA web site as a major no-no as well.

[Kozzie]

I like to use a hard rubber when shooting up around 400fps.

Prometheus red (hard) with a shredders nub, or a firefly rubber with a hard nub.