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mka 1919 lower replacement?


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now i really wish i hadnt cut at .600, hasn't happened to me yet though *crossing fingers

 

Receivers cut short will often fail when using the assembly bolt to draw the upper and lower together. If you fully seat the upper and lower together before tightening the bolt, you can alleviate some of the risk in that aspect.

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I am confused as to what is causing this. Is is it where the cut is made or design of certain adapters?

 

EDIT: Posted as the same time as Anubis above. What is is FB's recommended cut?

Edited by ec4321
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Jim

I havent cut my buttstock off my1919 lower..

Have you had or heard of a lower receiver breaking from an uncut bittstock? Would i be better off leaving it stock to sell as im on your

F.B. Alloy lower list

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now i really wish i hadnt cut at .600, hasn't happened to me yet though *crossing fingers

 

Receivers cut short will often fail when using the assembly bolt to draw the upper and lower together. If you fully seat the upper and lower together before tightening the bolt, you can alleviate some of the risk in that aspect.

Definitely fall into this "boat", my bolt also doesnt line up perfectly with the upper and it has to have some force applied in order for it to work

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I am confused as to what is causing this. Is is it where the cut is made or design of certain adapters?

 

EDIT: Posted as the same time as Anubis above. What is is FB's recommended cut?

There are two issues here.

 

The polymer is a fiber filled material, it is quite strong in compression loads, less so in tensile, and very weak in shear loads.

This material is NOT ductile, and will fracture if it is deformed.(forced to bend)

 

When your cut for a buttstock is made the material left has to take the load imparted by the bolt pulling the upper back into the lower at assembly.

When that small amount of material is insufficient to carry that load without bending, it will fracture.

 

Joining the upper and lower completely before tightening the bolt, can reduce that type of load on the lower.

 

The second issue has to do with buttstock adapters, and the relationship between them and the placement of the bolt hole in the upper.

An adapter that has a fixed hole and counterbore for the bolt will align only with the placement of the bolt location in the upper.

If that hole happens to align, It works fine.

 

If the hole in the upper happens to be a little high on the receiver lug, the adapter will follow the bolt hole and force the top of the adapter into the top of the cavity in the lower, this will fracture the lower, It actually takes very little error as the polymer will not deform at all.

 

 

When I designed our adapter, I used the rectangular cavity as the feature that aligns the adapter into the lower.

That method gave me the best cosmetic fit to match the lower.

The alignment of the bolt and the known variances in location of the bolt in the upper were accommodated with a slotted hole in the adapter.

 

As for the cut....I would recommend no less than .900 but people can decide that for themselves.

 

A little more info to take into account

 

The XN lower is weaker in the buttstock section than an early gun.

The rectangular cavity in the lower can vary in location, angle and internal size. (both XN and early) so fitting may be required for ANY adapter.

XN guns upper to lower fit can be very tight and even leave a gap at the rear, fitting the upper to the lower may be REQUIRED before cutting for an adapter, Failure to do so can result in the lower fracturing or even the front section of the upper breaking.

 

I could write a book with the details learned across hundreds of examples of this gun, but this should be good for now.

Jim

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Jim

I havent cut my buttstock off my1919 lower..

Have you had or heard of a lower receiver breaking from an uncut bittstock? Would i be better off leaving it stock to sell as im on your

F.B. Alloy lower list

I have seen one XN gun with a VERY tight upper to lower fit in stock form, fracture the lower.

It was replaced by RAAC.

 

I cant tell you to cut or not to cutlaugh.png

 

The Tac-12s are in full production now and I expect about 100 more to be done and back here in two or three weeks so it wont be a long wait.

Jim

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im not going to name the company for whos adapter it was because i dont want to give anyone a bad rap but i was having problems with the screw holes lineing up for a perfect fit so thanks for the info i never thought about the small amount of error causing a problem like this and cut it to the specs that were recomonded but is was the slights bit off so now i need to fix the problem i caused thanks for the info gentlemanjim

 

thanks for the offer ToysRUs but ill try my luck getting a aluminum lower if something changes ill try getting a hold of you.

I'd say that's our adapter, the crack/break could have come from a number of reasons, one of which is the variations of hole placement on the receiver. We have made some changes to our stock adapter to remedy this problem in the future, seems the XN version is the worst for consistent hole placement. Also the injection mold has changed some as well.

 

We have over 600 adapters out there many, including my gun , with over 2K rounds with no problems.

 

We never know exactly what Akdal will change next, most changes are for the better

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Good responses as always Firebird and Tooth. You guys are an asset to this place :)

 

I don't think the .6 cut adapters will work with a bigger cut, you would have to get a longer bolt for one, and the adapter and the upper mount section would never mate up at that distance, or am I stupid? The one I have would be worse with a .9 cut given the facts about the material of the lower.

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I am confused as to what is causing this. Is is it where the cut is made or design of certain adapters?

 

EDIT: Posted as the same time as Anubis above. What is is FB's recommended cut?

There are two issues here.

 

The polymer is a fiber filled material, it is quite strong in compression loads, less so in tensile, and very weak in shear loads.

This material is NOT ductile, and will fracture if it is deformed.(forced to bend)

 

When your cut for a buttstock is made the material left has to take the load imparted by the bolt pulling the upper back into the lower at assembly.

When that small amount of material is insufficient to carry that load without bending, it will fracture.

 

Joining the upper and lower completely before tightening the bolt, can reduce that type of load on the lower.

 

The second issue has to do with buttstock adapters, and the relationship between them and the placement of the bolt hole in the upper.

An adapter that has a fixed hole and counterbore for the bolt will align only with the placement of the bolt location in the upper.

If that hole happens to align, It works fine.

 

If the hole in the upper happens to be a little high on the receiver lug, the adapter will follow the bolt hole and force the top of the adapter into the top of the cavity in the lower, this will fracture the lower, It actually takes very little error as the polymer will not deform at all.

 

 

When I designed our adapter, I used the rectangular cavity as the feature that aligns the adapter into the lower.

That method gave me the best cosmetic fit to match the lower.

The alignment of the bolt and the known variances in location of the bolt in the upper were accommodated with a slotted hole in the adapter.

 

As for the cut....I would recommend no less than .900 but people can decide that for themselves.

 

A little more info to take into account

 

The XN lower is weaker in the buttstock section than an early gun.

The rectangular cavity in the lower can vary in location, angle and internal size. (both XN and early) so fitting may be required for ANY adapter.

XN guns upper to lower fit can be very tight and even leave a gap at the rear, fitting the upper to the lower may be REQUIRED before cutting for an adapter, Failure to do so can result in the lower fracturing or even the front section of the upper breaking.

 

I could write a book with the details learned across hundreds of examples of this gun, but this should be good for now.

Jim

 

 

Awesome information. Love the engineering information behind this.

 

Thanks

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Good responses as always Firebird and Tooth. You guys are an asset to this place smile.png

 

I don't think the .6 cut adapters will work with a bigger cut, you would have to get a longer bolt for one, and the adapter and the upper mount section would never mate up at that distance, or am I stupid? The one I have would be worse with a .9 cut given the facts about the material of the lower.

They will fit the .900 cut, but you may need to slot the hole and counterbore, and it would use a 25mm bolt

Jim

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This scares me. I have this adpater and my lower is cut to .605. I did however change the angle on the adpater so it sat @ 90• with the rail on the upper. When I reassemble the upper and lower I seat them completely then draw the screw in til it's tight. Other problem I'm having is the bolts that hold my DPH stock to the adpater are diggin into the lower.

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This scares me. I have this adpater and my lower is cut to .605. I did however change the angle on the adpater so it sat @ 90• with the rail on the upper. When I reassemble the upper and lower I seat them completely then draw the screw in til it's tight. Other problem I'm having is the bolts that hold my DPH stock to the adpater are diggin into the lower.

can you just shorten the bolts a little?

jim

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What do you do about the serial number?I wouldn't want my serial number just wondering around.

I engrave the lower as recycled part, serial # invalid

 

The original # is NOT removed! and I keep absolute track of where they went.

If LE ever had a question, they should contact us

There are many gun parts that carry a serial # that are not the controled component that constitutes the firearm.

this is one of those cases.

Jim

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  • 4 weeks later...

Mine just broke also. It's an XN and only has 128 rounds through it! Just got on the waiting list for a FB lower. Sucks to be down after all the money I just sank into this thing. Was fun while it lasted !

o snap sorry you had to join the party but on the up side it just gives you more time to load up on ammo

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Mine just broke also. It's an XN and only has 128 rounds through it! Just got on the waiting list for a FB lower. Sucks to be down after all the money I just sank into this thing. Was fun while it lasted !

Where did you make your cut? .600 or further back?

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Mine just broke also. It's an XN and only has 128 rounds through it! Just got on the waiting list for a FB lower. Sucks to be down after all the money I just sank into this thing. Was fun while it lasted !

As you can see from my post count I am a newbie to the MKA, however I did cut mine at just over .900 and it still cracked! Just wanted to say I have talked to Nancy at Firebird and have to say she is great to talk to. Very nice and a pleasure to deal with. I'm on the waiting list for a lower as in the previous posts of mine and to be frank.....buy a lower from them and don't cut your stock. It may work for some but it's not worth the irritation IMHO!!! I won't say who's parts I have because I don't want to give a wrongful accusation. My cut at .900 was my decision and not the instructions so that will not give it away. In the beginning before the FB lower was available in quantity I can see why you would want to do the mods, that why I did it! The reason for this post is to maybe push someone who may be on the fence as to what to do. I do not wish to slander anyone or any product!!!this was just my issue and how I delt with it. Thank you again to Nancy at FB for the customer service.

 

#172

On the list.

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