Plum Crazy Full Auto conversion with FTF's. Any Advice would be appreciated.

I converted a Plum Crazy last Thursday and am experiencing FTF's with light primer strikes on the rounds.

1) Used Bushmaster BCG, Trigger, Auto Sear, disconnector, and Hammer.
2) I used the Plum Crazy hammer spring that came with the lower.
3) Used the Plum Crazy trigger spring that came with the lower.
3) Using my Bushmaster upper I have had for several years.
4) I used a Jig to drill the Auto Sear hole.

1)With the first magazine I was able to get about 3 rounds in full auto before it quite. I cycled another round and was able to get a semi round off.
2) I tested different batches of ammo from Hornady to Monarch brass and same results.
3) I verified the hammer spring is not install upside down and the legs are properly seated on the sides of the trigger pin.
4) Auto sear is functioning well and the sear spring is correctly placed in front of the selector and in the grove for it.

Do you think I should replace the hammer spring with another? I could pull a Bushmaster hammer spring from another lower and test. Are there stronger hammer springs to be purchased and if so where?
What are your thoughts on additional issues that could be causing this?

Thanks,

Holto01

FFL 07/02

WTD, LLC

My guess would be that the PC hammer spring is too light for the heavier steel hammer, since the original PC hammer is polymer.

ETA, I wonder how long the sear pin hole will last in a polymer lower before becoming an oval...

Good question on the sear pin hole. I plan on milling out the rest of my conversions but having the Plum Crazy sitting there was tempting. I plan on testing the Plum Crazy to see what kind of wear and tear it will take over time. It the lower holds up with better internals and a good upper it could be a good alternative to build a lighter, less expensive sample.

Anyone else have and wear and tear data on the Plum Crazy lowers?

Stupid but often overlooked and accidentally done wrong: make sure your rear facing legs on your hammer spring are on top of the trigger pin, not below it.

Upgrade the hammer spring

Hammer spring legs are on top of the trigger pin. I will upgrade the hammer spring and test it later this week.

Stronger hammer spring didn't work. I am thinking either bolt bounce or timing. The plum crazy has a carbine spring and buffer and the full auto bolt carrier weighs more than a semi auto bolt carrier. I do have a heavier 9mm buffer but I don't have a scale. Should I try it first and then maybe an A2 buffer spring.

I did see the link on RDIAS timing check but I am not quite sure where to put the feeler gauges. I believe they are stating put the gauges into the ejection port between the barrell head and the bolt and measure from there.

Any thoughts????

noob here but I thought a Dias needed a Semi BCG?

No. LL needs a very specific type of semi carrier. DIAS needs a full auto carrier.

That's where I put the shanks of the numbered drill bits I use to check timing - against the face of the barrel extension, where they'll clear the bolt lugs, and hit the face of the carrier on close.

Standard centerfire RR timing is .100" +/- .020" Longer timing can give carrier bounce time to rear its head, while shorter timing can result in partial hammer follow, and weak primer strikes.

This is a full auto setup with GI Auto Sear. I was just referring to the RDIAS timing because there is a good article on testing your timing with a RDIAS and the same applies to a GI auto sear.

Any thoughts bolt bounce being the problem?

You said you checked the location of your auto sear? Or just that you used a template to drill it? A quick check is that the selector on "semi" should be pointing right dead center at the middle of the auto sear pin above it. It's the first thing I check when looking at a non-factory RR conversion experiencing issues.

You're in a new place here, with the plumcrazy lowers. Given the relative strength of the material, I might even lean toward receiver flex pushing stuff out of alignment... not an issue in a semi, generally, but significant in machineguns. If you have the capability to take vid, especially hispeed, of the rifle while firing it, and play it back slow you could assess that better.

However, if the problem is carrier bounce, the general remedies are heavier spring, and heavier buffer. Most potent combo would be a wolff extra power return spring and an H3 or tungsten buffer, though many work up to those through H, H2 first. A cyclic rate reducing buffer like the AAC or MGI can help with carrier bounce, too.

I did use a jig to drill the auto sear hole.

Today I took it again to the range with a heavier 9mm buffer. I am shoot the rifle in semi all day long. As soon as I switch to full auto I get 1 shot off and then the FTF. I also changed the BCG to another bolt/BCG from another manufacturer to see if that would make a difference.

I also took a video camera and set it up on 240 FPS to see if I am getting bolt bounce or if this is a timing issue. I am beginning to think it is a timing issue.

Here is the video I took. Please look at it and let me know if you think you are seeing bolt bounce.

http://www.youtube.com/watch?v=KQ-l2-099y8

Any other recommendations?

Hmmmm, I'm not seeing any bounce... How's the sear spring? If it's binding or weak, it could cause this problem...

Maybe its just me but in that video it looks like the auto sear is a little too far back.
Maybe its just the video making it look that way.

To my eye, that auto sear pin looks too far back, too. Which would give you advanced sear timing issues...

Like I posted earlier - when you flip the selector to semi, the selector should be pointing dead center of the auto sear pin, and that one doesn't look correct. Early release of the hammer will cause it to ride the bolt carrier home, and not have enough energy to transfer to the firing pin to light off the primer of the next round, which EXACTLY fits the circumstances of the malfunction you describe.

Double check your release point with numbered drill bit shanks. Proper timing is .100 +/- .020. Or don't check - if it looks off by eye in a video, it's probably more than far enough off to cause problems.

Compare your lower to this photo of a factory M16 (from another thread in this forum). The selector hole center may not exactly coincide with the boss on the left-side selector casting, but it seems blatant that your auto sear pin is further back than it should be.



And here's a PWA post-sample of mine, showing the selector-auto sear location I look for in conversions



You're a Class 2, chalk this one up to first-time conversion inexperience, and get another lower to convert. There may be dimensional issues on the PCF lowers which mess with the bowers (or whoever did yours) jigs. The center of the selector hole in relation to the hammer and trigger pins is not essential for proper semi-auto function, but the relation of the auto sear hole to the hammer and trigger pins is definitely essential to proper full auto function.

If necessary, refer to an M16 blueprint to locate your sear hole in relation to the hammer and trigger pins, rather than rely on a jig or template that uses what may be an incorrectly-located selector hole as its basis.

Thank you. I will get on this tonight. I used another jig and not a bowers. I was playing around and trying to measure the timing and I was thinking it was releasing early and following the bolt. Fortunately the PCF lowers are fairly inexpensive.

I'll let you know and take some close up pics on the sear pin hole.

Thanks again,

Torrey

Here are the images of the Plum Crazy lower with the Auto Sear pin hole. I used a jig I purchased from Gunbroker and here is a link to it http://www.gunbroker.com/Auction/ViewItem.aspx?Item=261988250.

I am not sure if the bowers jig looks like this but let me know or please show a pic if you think the bowers jig will perform better and I will get one.

The Colt pic above appears to be directly above the sear pin hole vs my pic and also the pic below the Colt.

Your autosear hole is definitely not where it should be.

The drilling fixture in that auction registers only on the selector hole. A fixture which does so is pretty much useless, as the autosear hole must be placed corectly in relation to the selector, trigger and hammer pin holes. A correct jig does just that –– registers on several holes, to preserve the correct geometry between the different components.

The last Bowers jig I saw (years ago) registered on all the holes. I don't know if the Bowers jigs still do so, but the easy solution is to give Tom a call and ask him.

I resized and rotated the images to show the relative sear position based on the selector body. Yours isn't far off (if it were, the auto sear wouldn't be able to catch the hammer at all), but it's definitely further back than the Colt factory location, or my own post-sample PWA.



The Colt is clearly slightly forward of center, while my PWA is very nearly centered, but every so slightly forward of center, and your PCF is nearly centered, but every so slightly aft of center. When you drill your next one, I'd recommend you err towards slightly forward, and do your best to duplicate the relative location of the factory Colt.

Guesstimating dimensions from the known auto sear pin diameter, I'd say you're about .040" further back than the Colt, while mine is probably about .010" further back than the Colt.

I have to ask, did you mill out enough meat for the sear selector leg to clear the reciever wall and not bind or get hung-up?

I have used the Bowers Jig and it works fine. I even used a DeWalt drill and "eyeballed" it. Works 100%. There is a lot of acceptable play.

Light primer strikes are either bolt bounce or trip being released to soon or bolt follow. If you get light primer strikes on semi-auto, then it is not trip too soon since on RR it is rotated out of the way. In this case the problem it is either bolt bounce or is hammer not being caught by the disconnector and it follows the bolt home.

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