Cesiumsponge
07-07-07, 19:59
Last time I posted anything shotgun related, it was on a Surefire 618FA unit with a KL5 head swap, which worked wonderfully. I didn't see anyone try it before so I decided to experiment and it worked out awesome. That thread is one page back.
One problem that bugged me about the 870 aftermarket is a lack of ability to mount a set of iron sights for anyone using a "high" tube style adapter for aftermarket M4-style stocks. With the "low" tube M4 adapters, you can still use factory irons (or aftermarket ghost rings), but not with the high tubes. Oddly enough, most of the high tube kits give you an upper rail which is great for an optic, but it doesn't give you provisions for irons which I also like to use. One of the tricked out factory FN shotguns DO come with an elevated AR15 style front sight post, but there are no aftermarket offerings for any platform to my knowledge.
After scouring around for the easiest method, I finally found a way to mount a front folding sight with an off-the-shelf item and minimal work to modify it. This is what she looks like:
http://www.impulselabs.org/Personal/Eric/Gun/side_1.jpg
http://www.impulselabs.org/Personal/Eric/Gun/side_2.jpg
The lifesaver: Armalite AR-10(T) SST EA1154KIT rail gas block. This is the ONLY Armalite gas block with a rail that can be made to work as it is .875" in diameter. I am not aware of any other .875" dia gas block with an integral rail on top of it. Any gas block in .308 bull barrel (.940) will be too large and any regular AR15 will be too small. (.750 and smaller). The downside is that it's stainless so you'll need to finish it unless you have one of those shiny marine 870s.
Closeup of business end, looks almost factory!:
http://www.impulselabs.org/Personal/Eric/Gun/gas_block_1.jpg
http://www.impulselabs.org/Personal/Eric/Gun/gas_block_2.jpg
http://www.impulselabs.org/Personal/Eric/Gun/gas_block_3.jpg
With this installed, I can mount any gas-block rail-height front BUIS while using a rear BUIS on a receiver rail (my case, KAC 870 RAS). It gives me a 22.75" total sight radius which should be accurate enough for slugging. I don't know if I'm the first to think up this idea but I haven't seen any DIY or commercial products like it... but maybe that just means I haven't come across an idea that already exists. The solid Cavalry Arms unit seen on my shotgun on the KL5 head thread was just too rushed and bulky; I wouldn't recommend using it (metal too thin which caused threads to tear out from shallow thread engagement on thin walls, too bulky, requires lots of filing for a $50 item)
Installed goodies:
2nd gen KAC 870 RAS
(added) Magpul M93B (until UBR comes out)
(added) Magpul MIAD
(added) Mesa Tactical high-stock adaptor
(added) Mesa Tactical adaptor-mounted 6-rd shotshell holder
Surefire 618FA, pressure & on/off (possibly Surefire M69 rail w/ remote light in future)
Surefire replacement KL5 LED head on 618FA
18.5" factory barrel with front bead delete required for rail gas block fitment.
(added) Armalite AR10T .875" stainless rail gas block, ID honed several thou, bottom surface contoured to curve of magazine tube, painted
(added) GG&G gas block front BUIS w/ tritium post
(added) GG&G rear BUIS w/ tritium dots
(added)Aimpoint M2 w/ quick mount (might upgrade to T-1 in the future)
Nitty Gritty How-to:
Grinding off the bead is easy and required if you are to slip a gas block over the barrel. You can probably torch it to remove the silver solder and bead but I'm nervous about applying that much local heat as it could change the barrel temper if one is careless since the barrel is entirely parkerized or blued so you couldn't tell if the steel was discoloring (at least I can't). I used a bench grinder, took my time, and managed to not discolor any of the exposed steel I was grinding so it was guaranteed to stay reasonably cool. I had a spare barrel just "in case" so I didn't have any qualms about taking a grinder to mine...just a thought.
It won't be pretty when you're done though, but the gas block covers up most exposed metal. I shot the exposed metal with some paint anyhow to prevent possible water from being trapped underneath at a future time so it should be okay.
Modifying the Armalite gas block is very easy if you have a mill and a honing machine. If not, its still relatively easy with a dremel, sandpaper, and some round forms to wrap sandpaper around; it will just be more time consuming. A power sander would make it go faster...it just takes elbow grease and repeated fitting attempts to get it right. The actual contour on the bottom of the gas block will expose some of the recessed shoulder for the two screws that tighten the block down. That's okay, I just put a matching bevel on the shoulder of the screws and it seemed to work out without interference. The whole thing should only take a couple hours if you work at it by hand.
If you are worried about it rotating around the barrel from heavy use...I wouldn't. That concave bottom I put on the gas block is required as it sits against the convex radius of the magazine tube and would not rotate at all unless you completely bent your tube magazine somehow, and a crooked sight would be the least of your problems at that point. Otherwise out-the-box, the bottom of the gas block wouldn't fit in the gap between the barrel and magazine tube without grinding in that contour. Along with the factory barrel clamp, I am certain the gas block fitment strengthens the tube magazine from bottom impacts since more of it's length is supported.
With the 870 RAS from KAC and holding a straight edge across the flat of the RAS, the barrel mounted gas block rail sits about 1/4" below the RAS, which is roughly the same height difference between a railed gas block and the A3/handguard rails on M4/AR15s so a gas-block BUIS works out nicely. However, different brand receiver rails for the 870 might have various heights so no guarantees but it worked out with the KAC 870 RAS and I would think there is enough height adjustment on gas block BUIS at that point to adapt to most of the aftermarket picatinny rails at the receiver.
One problem that bugged me about the 870 aftermarket is a lack of ability to mount a set of iron sights for anyone using a "high" tube style adapter for aftermarket M4-style stocks. With the "low" tube M4 adapters, you can still use factory irons (or aftermarket ghost rings), but not with the high tubes. Oddly enough, most of the high tube kits give you an upper rail which is great for an optic, but it doesn't give you provisions for irons which I also like to use. One of the tricked out factory FN shotguns DO come with an elevated AR15 style front sight post, but there are no aftermarket offerings for any platform to my knowledge.
After scouring around for the easiest method, I finally found a way to mount a front folding sight with an off-the-shelf item and minimal work to modify it. This is what she looks like:
http://www.impulselabs.org/Personal/Eric/Gun/side_1.jpg
http://www.impulselabs.org/Personal/Eric/Gun/side_2.jpg
The lifesaver: Armalite AR-10(T) SST EA1154KIT rail gas block. This is the ONLY Armalite gas block with a rail that can be made to work as it is .875" in diameter. I am not aware of any other .875" dia gas block with an integral rail on top of it. Any gas block in .308 bull barrel (.940) will be too large and any regular AR15 will be too small. (.750 and smaller). The downside is that it's stainless so you'll need to finish it unless you have one of those shiny marine 870s.
Closeup of business end, looks almost factory!:
http://www.impulselabs.org/Personal/Eric/Gun/gas_block_1.jpg
http://www.impulselabs.org/Personal/Eric/Gun/gas_block_2.jpg
http://www.impulselabs.org/Personal/Eric/Gun/gas_block_3.jpg
With this installed, I can mount any gas-block rail-height front BUIS while using a rear BUIS on a receiver rail (my case, KAC 870 RAS). It gives me a 22.75" total sight radius which should be accurate enough for slugging. I don't know if I'm the first to think up this idea but I haven't seen any DIY or commercial products like it... but maybe that just means I haven't come across an idea that already exists. The solid Cavalry Arms unit seen on my shotgun on the KL5 head thread was just too rushed and bulky; I wouldn't recommend using it (metal too thin which caused threads to tear out from shallow thread engagement on thin walls, too bulky, requires lots of filing for a $50 item)
Installed goodies:
2nd gen KAC 870 RAS
(added) Magpul M93B (until UBR comes out)
(added) Magpul MIAD
(added) Mesa Tactical high-stock adaptor
(added) Mesa Tactical adaptor-mounted 6-rd shotshell holder
Surefire 618FA, pressure & on/off (possibly Surefire M69 rail w/ remote light in future)
Surefire replacement KL5 LED head on 618FA
18.5" factory barrel with front bead delete required for rail gas block fitment.
(added) Armalite AR10T .875" stainless rail gas block, ID honed several thou, bottom surface contoured to curve of magazine tube, painted
(added) GG&G gas block front BUIS w/ tritium post
(added) GG&G rear BUIS w/ tritium dots
(added)Aimpoint M2 w/ quick mount (might upgrade to T-1 in the future)
Nitty Gritty How-to:
Grinding off the bead is easy and required if you are to slip a gas block over the barrel. You can probably torch it to remove the silver solder and bead but I'm nervous about applying that much local heat as it could change the barrel temper if one is careless since the barrel is entirely parkerized or blued so you couldn't tell if the steel was discoloring (at least I can't). I used a bench grinder, took my time, and managed to not discolor any of the exposed steel I was grinding so it was guaranteed to stay reasonably cool. I had a spare barrel just "in case" so I didn't have any qualms about taking a grinder to mine...just a thought.
It won't be pretty when you're done though, but the gas block covers up most exposed metal. I shot the exposed metal with some paint anyhow to prevent possible water from being trapped underneath at a future time so it should be okay.
Modifying the Armalite gas block is very easy if you have a mill and a honing machine. If not, its still relatively easy with a dremel, sandpaper, and some round forms to wrap sandpaper around; it will just be more time consuming. A power sander would make it go faster...it just takes elbow grease and repeated fitting attempts to get it right. The actual contour on the bottom of the gas block will expose some of the recessed shoulder for the two screws that tighten the block down. That's okay, I just put a matching bevel on the shoulder of the screws and it seemed to work out without interference. The whole thing should only take a couple hours if you work at it by hand.
If you are worried about it rotating around the barrel from heavy use...I wouldn't. That concave bottom I put on the gas block is required as it sits against the convex radius of the magazine tube and would not rotate at all unless you completely bent your tube magazine somehow, and a crooked sight would be the least of your problems at that point. Otherwise out-the-box, the bottom of the gas block wouldn't fit in the gap between the barrel and magazine tube without grinding in that contour. Along with the factory barrel clamp, I am certain the gas block fitment strengthens the tube magazine from bottom impacts since more of it's length is supported.
With the 870 RAS from KAC and holding a straight edge across the flat of the RAS, the barrel mounted gas block rail sits about 1/4" below the RAS, which is roughly the same height difference between a railed gas block and the A3/handguard rails on M4/AR15s so a gas-block BUIS works out nicely. However, different brand receiver rails for the 870 might have various heights so no guarantees but it worked out with the KAC 870 RAS and I would think there is enough height adjustment on gas block BUIS at that point to adapt to most of the aftermarket picatinny rails at the receiver.