Why Nubbins Break

[Russ]

I've broken more than my fair share of nubbins. "P" nubbins or "V" nubbins, I show no preference. I've seen the battered Superbolt that was gouged by a broken nubbin, and read RobAGD's response as to how he has mauled a Superbolt WITHOUT breaking a nubbin.

I believe the problem lies with the nubbin being pushed outward by the bolt when the gun cycles. I find that there's just not enough clearance in the mainbody for the nubbin to get out of the way of the bolt. This causes excessive wear on on the intersection of the "V", which leads to premature breakage.

As you can see in the pic, when the bolt contacts the nubbins, it pushes them out past the outside diameter of the barrel. I've found that some nubbins have had very severe wear after as few as 1000 rounds fired.

What I had done to prevent this, was to re-shape the "V" section of the nubbin, making the "V" a more obtuse (sp?) angle, then bending the short end back to being parallel to the rest of thr nubbin (follow me?)

The plastic nubbins, those that were given out by AGD, seem to have eliminated most of the problems associated with the wire nubbins, although I did trim them to provide enough clearance when the bolt contacted them.

Both nubbins are brand new, out-of-the box. Barrel is a stock E-Mag, with the E-Mag's stock bolt.

Note how the nubbin on the left protrudes quite a bit out from the barrel. I think it would have a very short service life, if it wasn't altered.

Any comments or observations would be appreciated.

[cphilip]

I think thats why the old style short nosed bolt had less problems. Or was it my imagination? And this would not be the first time we reverted back to old tried and true to solve a new problem brought on by improvement. Solve one problem and create another? Happens all the time. I agree with the plastic nubbin solution but the short nosed bolt also needs to be considered too. A short nosed Super Bolt? Oh man!!! Tom now we are adding to "the list" again!

[Ityl]

I have never had a problem with a nubin. My friends have when trying to adjust them. I put the P ones in, leave then alone and they work fine. Through over a year and a half with a mag, many many cases of paint from tournies. I see the problem when people try to adjust the nubbin when shooting small bore paint through medium bore barrels. Just my opinion.

[Chaos]

ok everybody complains about breaking nubbins, and I see all the problems, but maybe its just me... I've never broken a nubbin. 6 mags, countless barrels, I got my first mag 4 years ago, and have owned a number of mags since. Still, never broke a nubbin... never even had a problem. I almost put mine in with the P tho.....

[cphilip]

I would be very interested to know the ratio of chronic nubbin problems to use or not of the Long Nosed bolts.

First one I ever broke was right after the install of a long nosed and subsequently its quick removal. So that is surely isolated and not very scientific sample at all. Just curious if it was a fluke...or is their a relationship?

[Russ]

I'm real glad for you guys that never break nubbins, I really am, but the fact remains that some folks are breaking nubbins, and, with me being one of them, I'm trying to offer some insight as to why this might be happening.

I don't want this to turn into some sort of battle between the nubbin-challenged and the nubbin-blessed. I'm just trying to figure out the mechanics behind it.

You never broke a nubbin? good for you. Post on another thread. Please don't insinuate that because you have never had a problem, that the problem doesn't exist. If you have broken a few? Let's find out why.

Long nose bolts, double-nubbin barrels, we might be on to something here.

[RobAGD]

Well I have my own thoughts on the P nubbins vs V's.

I have broken or rolled 9 nubbins in this last year.

Before that in the 8 + previous years I have maybe killed 9 total.

What changes in that time. 1 thing for me Long Nose bolts.

I think that its a few things.

Sloppy nubbin groves cut too deep in the barrels allowing the over round egde of the nubbin to get wacked by the bolt.

The long nose bolt hitiing the nubbing at a higher rate of speed. Witha short bolt you have the bolt slowing down when it hits the nubbin. With the long nose bolt its still moving fast when its contacting the nubbin.

With those 2 factors I think you have a perfect recipe for rolled nubbins.

What I have done to counter act that is to adjust the nubbing so you getting the more curved edge of the nubbin striking the bolt allowing it to ride up the bolt face better.

BlackVCG has his own thoughs on this as well... maybe he will post them..

-Robert

[jdrotert]

where can I get these plastic nubbins? (NT)

x

[Mild 7]

Funny, my nubin lasted me for 2 years...can it even break? Maybe I should play more.

[Dragoon]

Well I'm one of the nubbin blessed, but in reading the above some things come to mind...

Since the nubbins are made to AGD specs I would think that there is very little varience between batches. The bolt diamaters have not changed as far as I know. I think since these factors are fairly stable (other then long nose vs short) that the problem lies with the barrels. The thickness of the barrel between the nubbin and the inside bore of the barrel may be the variable that causes the problems. Rob mentioned this above and it seems to me to be the area to look at first.

Does anybody know what this thickness is in a stock barrel vs some of the popular aftermarket barrels?

I may be way off, but that's my imput for now.

Douglas

[Russ]

The inside diameter of my mainbody is 1.005" the outside diameter if my barrel, measured at the breach, is .995". This leaves .01" clearance. The nubbin wire is .03". As you can see in the pic, the nubbin wire sticks out past the barrel. Seeing that there's only .005" per side clearance, the bolt just keeps smacking the nubbin until it wears, and weakens, resulting in breakage.

It appears that the Long-Nose bolt, while greatly reducing blowback, has created a new set of problems, i.e. excessive nubbin wear/breakage.

I see why AGD went ahead with the development of the plastic nubbins, Tom must have recognized the problem a while back, and has now found a suitable solution.

[XspyX]

Man, you guys are nutz. I've never broken a nubbin, and I've never had a problem with them chopping either?

[rhetor22]

I'm not sure how this would work out, the bolt probably rotates anyways, but could you get a dremel with a cutting disk and put a small groove in the bolt for the nubbin to go in, or would this cause problems?

[BlackVCG]

I spent a few hours during the Thanksgiving break to see if I could figure out what is causing the dramatic increase in nubbins breaking over the last year.

I've personally used the old style foamieless bolt up until August of this year when I got a few long nose foamieless bolts and a delrin bolt at IAO. Out of the 4 years or so using my old style foamieless bolt, I had only broke one nubbin, which was a V nubbin and it broke in my stock MiniMag barrel. The break resulted right at the point of the V and bent over the front end of the nubbin into the barrel and the other end seperated from the rest of the nubbin. I believe that it broke because I had it in there for so long without ever having changed it and over time the bolt wore down the point and then it snapped.

This is the reason why AGD went to the P-Style nubbins. The P nubbins have a larger surface area and don't wear as much as the V-Style. But it seems as though once the Warps and long nose bolts came out, the V nubbin became more popular because it was easily adjustable to prevent double feeding with the warp and P nubbins were starting to roll more often and it was less common with the V nubbin.

I have all but concluded that the reason why both V and P nubbins are breaking/rolling more with AGD long nose bolts is because there is no radius or chamfer on the end of the bolt about the circumference. With all old style foamieless bolts that I've looked at, they have a small radius. Having a radius on the end of the bolt helps because it creates more surface area to impact the nubbin and it's easier for the nubbin to slip off the end of the bolt.

Now if we look at P nubbins, the way in which they are shaped almost creates a flat spot on the backside where the bolt hits the nubbin. With the long nose bolts having no radius, they come to a sharp point. If you'll test it for yourself, stick a long nose bolt in a barrel with a P nubbin and clamp the bolt really tight in your hand. Pushing on it with a lot of force but very slowly (Hard to explain how to do this in words) you'll feel it snag on the nubbin. This is because the sharp point on the end of the bolt casing is hitting almost a flat spot on the P-nubbin. Using an old style bolt that has a radius, it slips by a lot easier and doesn't seem to snag on the nubbin.

Another reason why P nubbins are folding as much as they do is because they are not supported on the back side like V nubbins. The P nubbin does not have a portion that holds on the other side of the nubbin slot like the V-nubbins. This makes it so that it's very easy for the P-nubbins to bend in the barrel.

The reason why V-nubbins are breaking/rolling less is because the constant angle on the V creates a ramp that the bolt casing rides up as it's depressing it. But still, with the sharp point on the end of the bolt it's applying all of its force at one point on the nubbin, instead of dispersing it if the bolt had a radius on the end of the casing.

The only things holding me back from knowing for a fact that the problem is the lack of a radius on the end of the long nose bolts, is that I've only seen a few old style foamieless bolts, non of which were brand new. So I don't know for certain that AGD put a radius on the end of the casing. It's too uniform for me to think that it's just caused by normal wear from the bolt hitting the nubbin, so I'm assuming that AGD had a radius put on the old style bolts in some manner. I do know for certain that the new long nose bolts have NO radius or chamfer of any sort put on the end of the casing and this I believe is the one factor that is causing the dramatic increase in nubbin problems.

Now I'm trying to figure out a way to properly clamp a long nose bolt so I can turn it and use a piece of emery cloth to put a radius on the end of the bolt. You can't clamp on the base because it has the taper on it, and it's too short to clamp in a conventional lathe, so I'll figure something out. These are my findings so far from the time I spent testing things to see if I could come up with solid proof on why long nose bolts break/fold more nubbins than the old style AGD bolts.

[Russ]

BlackVCG,

Thanks for your input. I, too have noticed the lack of a radius on the end of the long nosed bolt. This, coupled with re-forming the "V" on the nubbins has help dramatically.

If your lathe has collets, try dropping the bolt through an 11/16" collet from the backside, then screw the collet into the collet chuck/holder. The nose of the bolt will protrude enough to polish the radius, and it will run true. That's how I did mine.