Spyder to Line SI Barrel Conversion Q

uv_halo · 03-19-2008, 10:47 PM

Hey any metalworker (lathe runners) types out there that could answer this?

Can either of the pictured M98 Freak Backs (1" O.D.) be turned down to 7/8" O.D.? I would say yes but, I don't know if the lettering or the slots would pose a problem for a lathe.

uv_halo · 03-20-2008, 03:46 PM

noxx55 · 03-21-2008, 09:42 PM

I would ask legion, but the lettering shouldn't be a problem. Not sure what the wall thickness is on a freak back though.

HP_Lovecraft · 03-21-2008, 10:13 PM

I remember asking Doc if he could fabricate a freak-back to those specs, and after comparing to his MegaZ, he determined there was enough room.

But fabricating from scratch, and modifying an existing back are 2 different things.

Murph · 03-21-2008, 10:48 PM

Too bad a freak kit would freeze right over on a mega Z. My loudener won't fit the bull barrel on my Z-1.

uv_halo · 03-22-2008, 09:35 AM

Each back has an overall O.D. of 1"

@noxx55, on the older style, shiny back, the overall wall thickness is 1/8th (the area holding the insert in particular). The breech end of the barrel has a lip that keeps the insert from sliding rearward which increases the wall-thickness at that end. The forward, tip threaded end is much thinner but, since that end doesn't need to be turned down, no issues there.

Cutting everything to the left of and including the O-Ring groove, turning down the next 2 7/8ths inches to 7/8 O.D., and putting a barrel retention screw groove in the turned down section will basically give you a Line SI Freak back. It should work because, the M98 Freak backs have that lip forward of the 98 threads that will hold the insert forward.

The groove for the barrel retention screw is 1/32" deep which would leave about 1/16th of wall thickness. This is a bit on the thin end but IMHO, but it will have a stainless insert and be inside the body of the Bushmaster so, I think it should be fine.

Chappy · 03-22-2008, 10:26 AM

Quote:

Originally Posted by uv_halo

The groove for the barrel retention screw is 1/32" deep which would leave about 1/16th of wall thickness. This is a bit on the thin end but IMHO, but it will have a stainless insert and be inside the body of the Bushmaster so, I think it should be fine.

With that thin of a barrel wall,I'd be worried about tightening up on the barrel retention screw. If you crank down on it, you could easily bend the freakback and pinch the insert in I'd think. And in my bushy/montneel experience, if you don't tighten that screw up good you'll find yourself barrelless at the wrong time.

Not sure how to comprimise those two situations, maybe a second barrel retention screw?
That issue aside, I'd think you'd be better off fabricating an original piece than modifying, but the m98 freak back seems like it would work based on your math.
Definitely interested to see your final results!

uv_halo · 03-22-2008, 04:04 PM

I'm a little bit concerned as well but, for perspective, my bushy armson pro barrel only has a wall thickness of 3/32 and the groove is still 1/32, leaving 1/16.

Now to find someone to do it :-)

uv_halo · 03-23-2008, 10:42 PM

For those who might be lost with all the numbers thrown around, here is a graphic:

A- Threaded Section including O-Ring Groove, cut off.
B- Turned down to 7/8"
C- 1/8"-wide groove turned down to 13/16"

Legion · 03-24-2008, 10:25 AM

I am sure it could be done. The 1/16 leaves me a bit leary. Let me look into it, i am a few weeks from being operational again anyway. If you are ok with the thin walls and aware that they will not have much structural strength.

The other thing to think about is that the barrel groove doesn't have to be cut to maximum depth, as long as there is enough groove for the retaining pin to hold the barrel i think it would be fine (correct me if i am wrong guys) that way more meat could be left on the barrel.

Might be smarter to make one out of stainless.

03-19-2008, 10:47 PM	#1 (permalink)
uv_halo Newbie Join Date: Mar 2008 Posts: 11	M98 to Line SI Barrel Conversion Q Hey any metalworker (lathe runners) types out there that could answer this? Can either of the pictured M98 Freak Backs (1" O.D.) be turned down to 7/8" O.D.? I would say yes but, I don't know if the lettering or the slots would pose a problem for a lathe. __________________ 1989 Line SI Bushmaster SI Deluxe, Left Hand Feed Last edited by uv_halo; 03-20-2008 at 12:14 PM. Reason: Clarification and correction of Title

03-20-2008, 03:46 PM	#2 (permalink)
uv_halo Newbie Join Date: Mar 2008 Posts: 11	Bump __________________ 1989 Line SI Bushmaster SI Deluxe, Left Hand Feed

03-21-2008, 10:13 PM	#4 (permalink)
HP_Lovecraft MCB Member Join Date: Aug 2006 Location: Southern Maine Posts: 2,971	I remember asking Doc if he could fabricate a freak-back to those specs, and after comparing to his MegaZ, he determined there was enough room. But fabricating from scratch, and modifying an existing back are 2 different things. __________________ www.montneel.com My Myspace nonsense "the evidence strongly suggests that neither Billy nor Adam (Smart Parts) could have invented the electronic paintgun" -Garr M. King, U.S. Judge

03-22-2008, 09:35 AM	#6 (permalink)
uv_halo Newbie Join Date: Mar 2008 Posts: 11	Each back has an overall O.D. of 1" @noxx55, on the older style, shiny back, the overall wall thickness is 1/8th (the area holding the insert in particular). The breech end of the barrel has a lip that keeps the insert from sliding rearward which increases the wall-thickness at that end. The forward, tip threaded end is much thinner but, since that end doesn't need to be turned down, no issues there. Cutting everything to the left of and including the O-Ring groove, turning down the next 2 7/8ths inches to 7/8 O.D., and putting a barrel retention screw groove in the turned down section will basically give you a Line SI Freak back. It should work because, the M98 Freak backs have that lip forward of the 98 threads that will hold the insert forward. The groove for the barrel retention screw is 1/32" deep which would leave about 1/16th of wall thickness. This is a bit on the thin end but IMHO, but it will have a stainless insert and be inside the body of the Bushmaster so, I think it should be fine. __________________ 1989 Line SI Bushmaster SI Deluxe, Left Hand Feed

03-22-2008, 04:04 PM	#8 (permalink)
uv_halo Newbie Join Date: Mar 2008 Posts: 11	I'm a little bit concerned as well but, for perspective, my bushy armson pro barrel only has a wall thickness of 3/32 and the groove is still 1/32, leaving 1/16. Now to find someone to do it :-) __________________ 1989 Line SI Bushmaster SI Deluxe, Left Hand Feed

03-23-2008, 10:42 PM	#9 (permalink)
uv_halo Newbie Join Date: Mar 2008 Posts: 11	For those who might be lost with all the numbers thrown around, here is a graphic: A- Threaded Section including O-Ring Groove, cut off. B- Turned down to 7/8" C- 1/8"-wide groove turned down to 13/16" __________________ 1989 Line SI Bushmaster SI Deluxe, Left Hand Feed

03-24-2008, 10:25 AM	#10 (permalink)
Legion Minion to Big oil Join Date: Jul 2006 Location: Woods Cross, Utah Posts: 1,156	I am sure it could be done. The 1/16 leaves me a bit leary. Let me look into it, i am a few weeks from being operational again anyway. If you are ok with the thin walls and aware that they will not have much structural strength. The other thing to think about is that the barrel groove doesn't have to be cut to maximum depth, as long as there is enough groove for the retaining pin to hold the barrel i think it would be fine (correct me if i am wrong guys) that way more meat could be left on the barrel. Might be smarter to make one out of stainless. __________________ http://jonesgunworks.net MY NEW SITE!!! Nelson upgrades and replacement parts. Halfblock conversion, halfblock parts,grips, Pump kits and custom mill work. Last edited by Legion; 03-24-2008 at 10:34 AM.

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