Converting 1-1/16"x16 threads to M27x1.5

s11033 · Oct 13, 2022

Hey guys,

I'm almost totally clueless when it comes to turning, so please bear with me...

I have an FN49 rifle that is threaded for a metric M27x1.5 barrel tenon. It fires an obsolescent caliber that I can no longer readily find surplus ammunition for, and I'm trying to make a new barrel for it in a more common caliber.

I've found that 1-1/16"x16 is a very common tenon thread for US-made rifles, including the Remington 700. I was surprised to find that the maximum OD of the M27x1.5 thread, according to this spec sheet, converts to exactly 1.062". That got me excited...

The problem is that the pitch is different- 16 versus 16.93 tpi. However, only about .7" of the Remington 700's .883" tenon is actually threaded. The rest is left unthreaded for a recoil lug. For my rifle, I need a .812" tenon, fully threaded. I'd be cutting off the first .071" of the Remington barrel so that the shoulder ends up where it needs to be. Part of my tenon would already be threaded, and the remaining portion would be turned to size, but not threaded yet.

Here's my stupid question of the day... Could I use an M27x1.5 die and start it directly on the 1-1/16"x16 thread to essentially cut a new pitch over the existing threads? By my calculation, I'd be taking about 5.8% off each thread. The threads at the beginning would be close to full form. The threads at the end (the portion that isn't already threaded) would be full form. In the middle, there would be some very thin threads, with the last one having as little as 30% of its form remaining.

I'm not worried about the strength of those threads. I'm confident that I can get sufficient torque on the barrel to retain it without stripping them. The only thing I really care about is whether I can do this with a simple die, or whether I'd need this done on a lathe anyway. If it would have to be done on a lathe, then I might as well start with a fresh blank and have someone either teach me, or thread it for me.

Cheers,

Steve

magneticanomaly · Oct 13, 2022

Your cut-over-with-a-die threads may or may not be strong enough. What they are very unlikely to be is true, that is, coaxial with and parallel with the bore.

s11033 · Oct 13, 2022

magneticanomaly said:
Your cut-over-with-a-die threads may or may not be strong enough. What they are very unlikely to be is true, that is, coaxial with and parallel with the bore.

Thank you, this is one of my major concerns. God I wish I just had consistent access to a lathe and to someone that know's how to run it...

john.k · Oct 13, 2022

There is a lot more to fitting a barrel than just 'cutting a thread".........you will also have to chamber the barrel,get the headspace set right ,cut extractor slots and any other features required,like gas ports in a FN49...........anyhoo,what obscure caliber is it in ,far as I know they were in 7x57 Mauser and 30-06.

Vecair · Oct 14, 2022

You don't say what caliber your FN is but they had at least five different ones supplied to many countries. This may be something to research, they made an adaptor to convert a 30.06 to fire .308 which I installed in my Springfield. Simple and easy. Possible you may find something like that or (with a lathe) make one.

s11033 · Oct 14, 2022

Yeah, I have pull-through finish reamers and headspace gauges for .308, 6.5x55, and a couple others. I probably built the first 6.5x55 Garand in the world!

If I went with .308, I'd probably set the Remington barrel back a few thousandths and finish ream with the pull-through. If I went with 6.5x55, I'd get a 6.5 Creedmoor barrel and use the pull-through. The FN49 also has interchangeable headspace shoulders like the FAL.

My barrel is 8mm Mauser. I didn't say it's obscure... Just that I can't really get surplus ammo anymore. I don't have any other rifles with 8mm bores, and I'm trying to avoid reloading for this one, since that would mean stocking yet another bullet diameter...

I'd have to turn a journal for the front sight base / gas block, and I've figured out that I can use the front sight base / gas block as a jig for drilling the gas port. I can probably skip the journal for the gas piston support for my use, but I could add it down the road if I find that I need it. I'm not sure how I'd do the extractor cut, but I figure it's probably not a very precise operation anyway.

The other idea I had was to chop an FN49 barrel and use it as a stub, ream it out, and press-fit a barrel into it. I don't know how I'd retain the pressed-in barrel besides an interference fit though. Maybe I could do a thermal shrink fit... But at that point it might just be easier to learn to cut threads...

john.k said:
There is a lot more to fitting a barrel than just 'cutting a thread".........you will also have to chamber the barrel,get the headspace set right ,cut extractor slots and any other features required,like gas ports in a FN49...........anyhoo,what obscure caliber is it in ,far as I know they were in 7x57 Mauser and 30-06.

s11033 · Oct 14, 2022

Vecair said:
You don't say what caliber your FN is but they had at least five different ones supplied to many countries. This may be something to research, they made an adaptor to convert a 30.06 to fire .308 which I installed in my Springfield. Simple and easy. Possible you may find something like that or (with a lathe) make one.

How do you keep your insert from popping out with your spent case? I read the Navy had enough trouble that they ended up just making new barrels for them.

Mine is an 8mm Mauser and I don't want to stock .323 bullets because I don't use them in anything else... It was a rehab project that blew up in someone's face due to a broken firing pin causing an out-of-battery detonation. I bent the bulged receiver and receiver cover back into shape, replaced the barrel and got lucky with the headspace, replaced the bolt carrier group with a later-style Belgian contract bolt with a firing pin blocking mechanism and two-piece firing pin, and I threw it into a Belgian stock that I refinished. It's a total mongrel, but it's become one of my favorite rifles to shoot, and has chewed really nicely through my dwindling stash of Yugo 8mm...

Now if I could figure out how to barrel it in 6.5x55, I think it would be the coolest FN49 on the block, and would go nicely with the Garand I did in that same caliber. With that one, I managed to convince Criterion that it was a good idea, and they made me a barrel on condition that I'd help them establish their gas port spec!

Steve

Vecair · Oct 17, 2022

Well I believe the theory is that the insert expands with the first shot to lock it in. But I have also seen where they put a bit of red Loctite on the outside, fire and let it cool and set before ejecting the shell. Probably not fool proof but if you made your own you could size it to be permanent I would think. Easy enough to experiment doing.

GregSY · Oct 17, 2022

Since you live in Plastic, it's hard to tell who might be in your area that could help you.

s11033 · Oct 17, 2022

GregSY said:
Since you live in Plastic, it's hard to tell who might be in your area that could help you.

Hey, didn't realize I didn't have a location up! I'm in Boston, but most of New England is within driving distance!

Steve

john.k · Oct 18, 2022

I once retapped hundreds of 24mm nuts to 1"UNC to save money..........the original plan was to make the 1" nuts ,but I found bags of the 24mm nuts for scrap price .......as the nuts were all part of assemblies ,the customer was never the wiser.

tdmidget · Oct 18, 2022

So, you can't get 8X57 but you can get 6.5X55 ammo? I would have thought otherwise. I used to have a model 49 in 7X57. It was a wonderful rifle. Traded it on a Mouser model 66 in 30-06. Wish I still had it.

TruckerJ · Nov 7, 2022

s11033 said:
Hey guys,

I'm almost totally clueless when it comes to turning, so please bear with me...

I have an FN49 rifle that is threaded for a metric M27x1.5 barrel tenon. It fires an obsolescent caliber that I can no longer readily find surplus ammunition for, and I'm trying to make a new barrel for it in a more common caliber.

I've found that 1-1/16"x16 is a very common tenon thread for US-made rifles, including the Remington 700. I was surprised to find that the maximum OD of the M27x1.5 thread, according to this spec sheet, converts to exactly 1.062". That got me excited...

The problem is that the pitch is different- 16 versus 16.93 tpi. However, only about .7" of the Remington 700's .883" tenon is actually threaded. The rest is left unthreaded for a recoil lug. For my rifle chassis, I need a .812" tenon, fully threaded. I'd be cutting off the first .071" of the Remington barrel so that the shoulder ends up where it needs to be. Part of my tenon would already be threaded, and the remaining portion would be turned to size, but not threaded yet.

Here's my stupid question of the day... Could I use an M27x1.5 die and start it directly on the 1-1/16"x16 thread to essentially cut a new pitch over the existing threads? By my calculation, I'd be taking about 5.8% off each thread. The threads at the beginning would be close to full form. The threads at the end (the portion that isn't already threaded) would be full form. In the middle, there would be some very thin threads, with the last one having as little as 30% of its form remaining.

I'm not worried about the strength of those threads. I'm confident that I can get sufficient torque on the barrel to retain it without stripping them. The only thing I really care about is whether I can do this with a simple die, or whether I'd need this done on a lathe anyway. If it would have to be done on a lathe, then I might as well start with a fresh blank and have someone either teach me, or thread it for me.

Cheers,

Steve

8mm is not obscure, but yeah, hard to find. As john said, it's sadly not as easy as just cutting the thread, a lot more comes on top when you switch calibers. Maybe it's easier to trade/sell the rifle or get into reloading so you can "make" your own 8mm

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