What is Supposed to "Stop" / "Bottom Out" the Pump Stroke on a Sniper?

[Booska]

The local field is flooded this weekend and I'm stuck inside with some silly questions to ask on MCB.

What is supposed to "stop" a pump stroke on a full block sniper? Said another way, what determines the farthest back that the pump handle can be?

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I see 4 possibilities for this:

1. The pump plate hits the base of the pump guide rod.

This doesn't seem great because you have an anodized aluminum pump plate banging up against a stainless steel guide rod. This is bound to screw up the anno right?

2. The cocking rod is far enough back that the hammer lug hits the back of the lug slot on the body.

This also seems bad because you have a lug likely made out of stainless steel banging against the back of the lug slot which is aluminum..

3. The pump return spring becomes fully compressed and prevents the handle from moving farther back.

This seems good to me.

4. The tip of the guide rod bottoms out on the guide rod hole in the pump handle.

This also seems good to me.

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What I originally had on my latest build:

I recently built my first sniper from scratch and I didn't think too much about this topic. I ended up having a case of #2 from above without realizing.

One day out on the field I was pumping away and my hammer started getting stuck in the lower tube.

I ended up finding that the hammer lug was smashing the back of the lug slot so much that the aluminum deformed and pushed some material up into the lower tube. As a result, my hammer would get stuck when I pulled the trigger and wouldn't strike the valve.

I was able to fix this by taking a metal file to the back of the lug slot and cleaning up the material that had gotten pushed up into the lower tube.

Here's a picture showing what happened:



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Where I'm at now:

I ended up cutting a 1/4" diameter wooden dowel and putting it in my pump handle's guide rod hole. I made it the perfect length so that the pump stroke would bottom out via the guide rod tip hitting this wooden dowel (basically #4 from the list above). This happens right before the pump plate hits the base of the guide rod.

This seems to work really well and it's allowed me to adjust everything else freely without needing to worry about this issue.

I can now pull the pump handle fully back, then grab the cocking rod and pull it another 1/4" or so. This confirms that the lug is no longer damaging the back of the lug slot.

I'd like to try this with rubber dowels at some point to see if that feels a little bit better.

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Questions:

1. Is this a common issue? Or was I somehow stupid enough to create a new problem? I don't see this kind of thing discussed anywhere online. Are there other people out there destroying the back of their lug slots?

2. Is there something else I'm missing? Maybe a better method than what I mentioned above (wooden dowel in pump handle)?

EDIT / SUMMARY:

Based on discussion here, the most normal way of going about this is having the hammer lug hit the back of the lug slot to stop the pump stroke. Ideally, you're rarely ever hitting this point because you should only be pumping until the sear catches to avoid extra unnecessary travel.

In my case, my sear catches even the tiniest amount of hammer lug. This allowed the hammer lug to sit very shallow in the lug slot and as a result, it chipped away at the very top of the lug slot rather than hitting the lug slot nice and square.

I ditched the wooden dowel in the pump handle trick in favor of a properly adjusted hammer lug adjustment that sits deeper in the slot. Now I get nice square contact for those unideal moments where I get out of rhythm and pump it a bit too far until the lug hits the back of the slot.

Anyways, this was a very specific / weird question, but I'm glad I asked it. Always something to fine tune / learn.

[Cunha]

I put a piece of brass dowel inside my pump handle cut and filed to my preferred stopping point.

Stock setups are the cocking lug bottoming in the slot.

[Booska]

I put a piece of brass dowel inside my pump handle cut and filed to my preferred stopping point.

Stock setups are the cocking lug bottoming in the slot.

Ah good to hear. Sounds like I'm on the right track then with my wooden dowel in the pump handle.

Maybe I only had a problem with the lug bottoming out because it was just barely turned in enough to grab the sear. So the lug was just barely contacting the top of the back of the lug slot.

Maybe it would have been fine if I turned the lug in another turn or two and it was hitting a more solid part of the lug slot.

[Chuck E Ducky]

If your lug is set to high the trigger often feels spongy and won’t engage consistently. I adjust for early engagement on non Autotrigger markers as it takes trigger play out. For AT I adjust for the 1/8” gap trip. You can get a lil bit closer without AT.

[iamthelazerviking]

Pump arm length. I had a pump arm that was too short, pump handle hit the plate. Longer pump arm helped with that. Sear caught the lug earlier, without handle hitting plate

[Booska]

Good point. A longer pump arm could fix #1 above, but then I guess you end up with case #2 (lug hitting back of slot).

Based on what Cunha said, that might just be the norm though. Maybe I had a freak accident with mine.

[Chuck E Ducky]

Too long of a cocking rod can cause that as well especially with a shorter pump rod. I look to adjust the marker to cock just as the bolt clears the back side of the feedneck hole. Well adjusted and tuned pumps and cockers do this.

[Socalpumpballer]

Cocking rod and lug slot is the answer. If you’re deforming the lower tube on a sniper by cocking it. Then that’s user error causing a problem that shouldn’t exist. As far as fine tuning any over travel. That can be done a number of different ways. If you have the old aluminum guide rod plate just throw an oring over the main rod. That being said your pump arm shouldn’t be so short that it’s bottoming out.

[Chuck E Ducky]

Yeah if you are blasting that slot up you are pumping it way too hard for one. Your cocking rod is likely set to long and your pump arm to short. You have a lil adjustment at the back block as well. If you have to pump it that hard to get it to cock it could be that your hammer lug is set to high. Try 1/4 turn down at a time till it cocks earlier. If you have A/T you want it to fire 1/8” before the back block touches the body.

[Grendel]

Sear catch, cocking rod length and muscle memory.

[Booska]

Yeah if you are blasting that slot up you are pumping it way too hard for one. Your cocking rod is likely set to long and your pump arm to short. You have a lil adjustment at the back block as well. If you have to pump it that hard to get it to cock it could be that your hammer lug is set to high. Try 1/4 turn down at a time till it cocks earlier. If you have A/T you want it to fire 1/8” before the back block touches the body.

Well I was coming from a phantom with a red spring, so definitely possible I was strong arming it a bit. But I'm also playing with a 12rd feed and generally taking my time. I'm not out there slamming an AT at 5bps like I'm trying to get an arm workout in haha

I think the bigger problem was that the lug was barely into the slot to the point that it was chipping away at the "top" of the slot.

I'm not running an AT, so I don't need to worry about timing an AT with the lug depth.

I turned my lug in a couple more times and no longer had issues because the lug was making contact with a solid part of the back of the slot (rather than chipping away at the top of the slot).

I'm running an S6 length rod with a CCM EZ kit and a Mozak full block body. Justin told me he usually runs the same pump arm.

Ive tried going up a size in pump arm length, but then it's a bit too long and I find that there's like a 1/2" of play in the pump stroke before the pump return spring kicks in. That 1/2" of play is a bit annoying because just pointing the marker up in the air is enough for gravity to pull the pump arm back 1/2". With the S6 length, the pump return spring is just engaged a bit with the pump handle all the way forward. Keeps everything feeling solid.

I think the wooden dowel in the pump handle seems to be the best of both worlds.

My sear catches right as the bolt clears the breach and shortly after, the wooden dowel in the pump handle stops the stroke when it hits the tip of the guide rod.

What I've gathered is that the lug stopping the pump stroke is indeed normal. I think my situation was just caused by the lug depth being way too shallow.

[latches109]

^ plus set the cocking rod to the correct length. that should be no more than 1/8" past the breach. If you need more length on the pump arm then expose the threads on either side. try to keep a min of 5 threads on either side. And, do not use a cone lug. When properly setup with the correct parts, you should not need any rear travel stopping devices.

[Booska]

Yeah cocking rod and pump arm length adjustments were perfect.

My issue was just too few turns on the lug resulting in it being too shallow and chipping away at the very top of the lug slot.

As Chuck said, it wasn't hitting the back of the lug slot "square" which is a perfect description.

So yep kind of a setup issue on my part. I took the advice on a thread somewhere (maybe PBN or MCB) to turn the lug until it just starts to catch, and then turn it in another half turn.

That resulted in my lug being too shallow.

Silliness on my part - but hey it's part of learning I guess haha. Maybe this thread will help someone out some day.

[latches109]

this is where part fitment comes into play and details get lost. Running the lug depth a min of 50-100% flush with the bottom of the body is a good idea. Different sears sit at different heights out of the frame on different frames. I have had some combinations really sticking out and some where the sear barely clear the top of the frame. My guess is your sear sits really high out of the frame, meaning IF you want the hammer to drop early in the pull, this would cause a "shallow" lug depth. This is where having a mountain of parts (like me) helps, I can swap parts to fine tune where the trigger pull starts the hammer drops, while maintain the lug depth 50%+ in the lug slot. Most AT setups I am close to flush with the body by the time front of the bolt clears the breach for the hammer drop. just rambling hope that make sanse

[Cunha]

On a half block the bolt pin can bottom on the little piece of aluminum between the two tubes. This is very easy to beat up. So I don't recomend going without a pump handle or other stop on those.

With a stop in the handle you have a positive end to your pump stroke its easier to keep a good rhythm and it protects the body.

But everyone can do what they like.

[Booska]

I agree.

I'd assume ideally you're not bottoming out the pump stroke every time anyways.

It's more there for the 1 out of 10 pump strokes where you get out of rhythm and pump it a bit too far / hard.

[Cunha]

On an autococker it is usually the lug slot that stops the rams movement backwards.

On the cockers I used to use, I would make a stop in the ram for rear travel and of course adjust the back block to not touch the body.

[Jordan]

I have my brass midblock set up with a modified pump arm.

I shortened the arm just enough that the pump handle bottoms on the guide rods just after the bolt clears the feedneck/cocks the gun. Keeps the pump stroke short and brings the handle a bit closer to the body of the gun which makes it a bit more ergonomic for me.

I think I took half an inch off?