Need advice from you experts here.

Recently acquired a Bushy preban dissapator upper.  When I went to attach it to my Colt preban rifle, I was suddenly aware of the difference in front takedown pin geometry.

I see there are offset front pins to accomdate this situation.  But, it looks like the offset front pin requires front pin removal first in the takedown process, to prevent breaking the lower.  This would seem to make it impossible to quickly remove/clean/inspect the bolt carrier group by simple pulling the rear pin, and breaking open shotgun style.

Does anyone "open up" the small hole in the upper, to fit the Colt lower?  Do I have any options besides a offset front pin, or a different preban lower with the small holes?

Thanks in advance for your feedback.

MM

A small hole upper cannot be reamed out for a large pin Colt lower. If you will look carefully you will see that the boss on the upper is a bit rearward and if you were to ream it out, you will completely go through the front of the boss thus ruining the upper receiver.

It won't work.

If you'll buy the adapter pin sold by CDNN, and install it properly, you'll have a piece that works just like the original SP1 hinge assy. Nothing will break. Just install it w/the large head on the right side, and the small flat on the head facing to the rear.[:D]

[url]www.cdnninvestments.com[/url]

Or you can go this route:

[url]http://www.danieldefence.com/store/cart.asp?catid=1[/url]

They are in stock now. I'll be trying one out shortly myself.

I like this option.  Think I'll either go this route, or, see if I can find a complete preban lower with the smaller hole, and have a complete second rifle.

Thanks,  MM

Last I heard, these pieces had to be hand fitted, and were not drop ons.  That may have changed, but you might want to check w/the maker before ordering[:D]

Why couldn't you enlarge the small hole and create a large hole???

Granted the large hole couldn't be co-axial with the small hole, beside the large hole and small hole are offset just like the pins Bushmaster sells.

Any competent machinist could do it for you, then you would just need to have it refinished...

 This would be my favored/most economical route.  Was interested in a reference.  I wouldn't send it off to someone who had never done it before.  Don't want to be the guniea pig.

 The holes not centered on each other when I put the upper/lower together, but the large hole does completely surround the smaller hole.  So it is not as simple as enlarging the small hole on it's existing center.

MM

 

 

I used the CDNN adapter on my Colt/Bushy hybrid.  I am very satisfied with it.  Still requires screwdrivers if you want to switch uppers.  This is no big deal, as the original Colt has the same problem.  When the AWB sunsets, my 16" preban upper will probably stay on my postban lower, and the preban Colt will get its 9mm upper back.  Seems like I have one more upper than I do lowers, so I guess I need to buy another lower.

Decisions, decisions!

It is not that simple.

The problem is that the holes do not line up concentric with each other and when you drill out the upper's hole, it will go completely through the front of the existing boss that contains the hole. It will NOT work. All you will do is ruin an upper.

Well I think it's supposed to work if you machine it properly and do not use the original hole as the guide.  The Daniel Defense large hole upper is a Les Baer upper that has had the hole enlarged.  This is why minor fitting may be required with that receiver (yes it's true shamayim)

Actually, if you could use the smaller hole on the upper as a guide it could work by just simply enlarging it to .315". But, that is the problem, you can't. By the time you drill it out the way it has to fit, you simply go completely through the front of the hole completely through the boss. It won't work.

AK Mike, this is not how we make Big Hole Flat Top Uppers.  A Big Hole upper starts with the forging - some have enough meat for it and some don't.  Please do not try to convert a small hole to a big hole.  Even if you don't tear through the hole in the drilling process, you will be left with a dangerously thin front pin hole.  We get orders from guys with ruined uppers all the time.

southern

PS:  Our current uppers usually don't require fitting.

[b]Yes, in fact, it is that simple!!![/b]

Did you even read my post???  I stated that the holes aren’t concentric or coaxial or how ever you want to say it, but they don’t need to…

All you need to do is put the upper in the vise of a vertical end mill. And create a new hole with a center cutting end mill, period...  No you can't do it in your garage with a vise, a power drill, and a 8mm drill bit; but yes it can most definitely be done with great results...

Do you want to know how I know, I’m a mechanical engineer and I have a machine shop… I machine hole all the time that overlap with edges and other holds that are not concentric all the time. (Not gun related, engine related)

Personally I would use a 5/16” center cutting end mill and then use an 8mm (0.315”) reamer to make finish the hole so it is nice and smooth.



Bingo, we have a winner, AK Mike understands...

Again Rebel you don’t need a guide; it’s called an end-mill. Repeat after me end-mill, end-mill, end-mill, end-mill, end-mill…

Now as to whether or not there is room in the raw forging for the particular upper you have is another matter entirely.  Not all uppers are produced mil-spec and there is a lot of variance out there, but a competent machinist could measure that in about two seconds and know for sure.

Upon comparing my Bushmaster 16” M4gery upper to my Colt Sporter Match H-Bar upper; the front lug/boss whatever the proper term is, are the same size within a couple thousandths with the only difference being the holes machined into them...

I have no doubt home brew gunsmiths destroy their uppers all the time, but that doesn’t mean it can’t be done properly by somebody that knows what they are doing…

[b]YES IT CAN BE DONE[/B]

Troy,
According to your dimensions you would only have approx. 0.0205" of material on the bottom of the hole, that’s not good and if that is the way they all are then I admit it should not be done.

Although I thought my dimensions on my Colt and Bushy were the same for the boss, but I may be wrong. I'll measure it tonight and post the result.

The 0.315” center cutting end-mill would not be totally oddball, but totally common; 8mm = 0.31496". If you walk on the wild side you could simply get a 8mm center, or the more conservative way would be to use a very common 5/16" center (0.3125") to create the hole, and then finish it with a common 8mm reamer.

You say this would make a totally custom AR setup, like that is a bad thing?  There are many Colt big hole lowers out there and if a guy has a big hole lower presently, it would only make sense to have a big hole upper(s) to go with it right, or am I missing something…

Thank you, Troy for the dimentioned picture worth a thousand words.

southern

The height of my Bushy is 0.475"
The height of my Colt is 0.481"
- The Bushy converted would be 0.006” thinner than the Colt on the bottom.

The thickness of my Bushy is 0.450"
The thickness of my Colt is 0.475"
- The Bushy converted would be 0.0125” thinner than the Colt front & back.

With my particular combination I feel I could do safely do it, with the combo Troy posted it shouldn't be done; so I guess it depends on the particular small hole upper…

Southern, I stand corrected, that is just what I was told.  Could you clarify for me a little though.  Are they Les Bear forgings?  Are they forgings destined for small hole but are made large hole starting from the forging, allowable because those particular forgings have enough "meat" to begin with?  I realize they may not require fitting, which is why I used the word "may" as I gathered from the website.  I think the only one I know of did not require fitting at all.  If you make the large hole from the forging, why would they ever require fitting though?

P.S. I am patiently awaiting the next batch to be ready as I am contemplating a new upper for my large hole Colt.  Keep up the good work!

Mike, Les Baer did make our Big Hole Flat Top uppers for us.  On those, he handled the total process including the sourcing of the original forgings.  As I understand it the forgings he originally started with had enough meat to allow them to be used for big holes.  

We recently re-designed these uppers to improve the fit to a wider range of early Colt lowers and in doing that we re-sourced the forgings.  These latest uppers should exhibit equal or better quality with less hand fitting required for most lowers.  

southern

You may want to consider tightening up the specs on the next batch! I installed my DD upper yesterday and the fit is VERY loose. The large hole on mine specs out at .330 not .315! Universal fit is a nice thing, but .015 is far too much clearance for the front pin.

AGREED. If you move the hole and increase the diameter it may break out, unless the boss is forged the correct size and in the correct location to receive the larger hole.
--RR

"vertical end mill"??? How about a VERTICAL MILL. Using the word END when describing a vertical milling machine (such as a Bridgeport) means nothing. An "END MILL" is a type of milling cutter. The word END has no place in the description of the milling machine itself.


I'll go along with that, though there is no need for a "center cutting end mill". Since there is an existing hole you would only be cutting on the outer diameter of the end mill as you plunge in the offset location. A center cutting end mill is only needed when plunging into virgin material with no existing hole.


Another way to move and resize the hole is single point cutting with a boring bar. This would also provide a better finish than end mill or reamer.
--RR

Thanks for all the info guys.  Maybe after "sunset" I will fit this upper onto a Postban Oly I have, and call it good.

Thanks again,  Shane

Is there any functional difference between this CDNN adaptor pin:

[IMG]http://store3.yimg.com/I/cdnn_1756_12821363[/IMG]

and this DPMS adaptor pin:

[IMG]http://www.midwayusa.com/midwayusa/applications/mediasvr.dll/highresimage?saleitemid=316328[/IMG]
MM

YES!!! That small head POS that DPMS, and just about everyone else, sells will rotate w/ the upper when you open the action, but it won't rotate back when you close it; meaning that you have to get out the screwdrivers and reposition it every time you open the piece up.  The CDNN unit rotates both ways, so it stays in position, and once set up you don't have to ever mess w/it again unles you've removed it to take the two halves apart.

IF you buy the small head version you'll hate it---I promise you[:D]

Thanks Shamayim (grin)...  

I knew there had to be a reason the DPMS unit cost only half as much.

MM

Thanks for all the help.  I just ordered one of the CDNN pins.

MM

DPSM also sells a large head offset adapter pin.

http://www.dpmsinc.com/10Expand.asp?ProductCode=LR-OS2

However, they say that it is for mating large hole uppers to small hole lowers.
Thing is, it looks exactly like to CDNN unit, or am I missing something?

Mick

Another plus is that the CDNN pin prevents the user from tightening the wrong end by accident.

As a Colt preban owner (obviously large pivot) I gotta say, Only Colt could **** something so simple up so bad. Man, I love my Bushmaster.

Can't vouch for it's accuacy, but was told many years ago, by a Colt employee, that the reason they built the SP1 w/the large, screw type pivot, and smooth side upper, was that the push pin and forward assist had been designed by US Army Ordnance, and they didn't think they could get the OK to incorporate them into their civilian models.  Also, re the smooth side uppers, they had a bunch of raw forgings left over from the Air Force contracts and wanted to use them up.  He said they would have been happy to make the SP1s small hole push pin, or at least small hole, but didn't want to risk pissing off their largest customer; the US Army.

Now this is just an unsubstantiated statement from a production line worker I met out at a range in Ludlow, Mass, around 1966; but it does seem a reasonable explanation for the drastic difference in the front hinge assys of the two types.

Thanks for the help guys.  Got her put together with a CDNN pin, and went to the range with Federal American Eagle 55gr FMJ ammo.  Not a tack driver, but held under 3 moa, in 15-20 mph winds at 100 yards.

Heres' a pic of the end result:
[IMG]http://www.hunt101.com/img/077403.jpg[/IMG]