How to Remove OEM FNH Scar Three Prong Flash Hider (Scar 15P, DMR, Etc.) at Home Guide - Think I Found an Easier Way
I have read a lot of methods suggested and have a suggestion for how to hopefully take them to the next level to try this at home. AS ALWAYS PROCEED AT YOUR OWN RISK as there’s no guarantee this will work for you. There are some great folks in the forum including u/eMGunslinger that people highly recommend using to remove it if you can’t.
I think I figured out why these are so hard to remove: I believe FNH thermally fits the flash hiders on at factory (based off reading they thermally fit other parts and how well this method seemed to work) by using a temp differential (ie I think they put hot flash hiders on cold barrels and apply normal amounts of torque) so my method tried to reverse that and it seemed to worked twice. I just used this method to remove both my 300blk three prong (which would not budge at all with a cheater bar, wd40 and pure torque) and a friends 6.5CM DMR flash hider and that one rotated really easily following the method.
THE METHOD USED:
Prior to starting I added WD40 at flash hiders. I sprayed onto threads but if you have a lot, soaking is probably better. I left the 300blk to sit for half the day but for the 6.5CM I left for 15 min and heated the flash hider to be warm/slightly hot to touch to help the oil penetrate which I believe worked equally as well as waiting.
Next you need to cool down the barrel JUST NEAR THE END OF THE BARREL WHERE THE THREADS ARE. I suppose you could put the entire bbl in the freezer but I did not because I was worried this would risk making the barrel spinning out of the extension more easily. For the 300blk barrel I cooled it by cooling a spare piece of McMaster alignment rod in an ice bath and shoving shaved ice into the muzzle end with a dowel to compress the ice and cool the barrel near the muzzle end. You don’t need the rod, but it helped keep the ice compacted and in the right place. I repeated shoving and compressing the shaved ice into the bbl until the bbl end was cool and the ice stopped melting quickly. On the 6.5DMR it was even easier as I just stuck the muzzle into an ice bath first for 5 min and then shoved some shaved ice into there for maybe a couple min so it was cooled very quickly.
I clamped either barrel then into my vise at the extension with copper soft jaws to prevent marring and torqued the jaws tight to prevent it moving (keep in mind that the barrel can jump when the hider comes loose so make sure the lower aluminum pic rail (which I kept on) isn’t close to the jaws else you can marr it.
Next, use a torch to rapidly heat just the section of the flash hider that’s over the threads (the grooves right behind the tines). I had a family member do this while I manned the wrench. It took maybe a min or two of doing this while I applied solid and even pressure to the muzzle device with an AAC breakout tool attached to a 12” breaker bar to come loose. I formed a piece of a chipotle aluminum lid around the tines of the aac tool to take up slop and help not marr the flash hider and it helped. If you don’t have this tool another trick you can use if you are okay to marr/junk the flash hider after (or possibly the wrench if the ring end breaks) is to use the bottom ring end of a wrench and stick it through the tines and rotate that way. I didn’t try this this time but have used that method to remove/attach other 3 prongs in the past.
Other notes: There is no rocksett on the threads or anything I could tell that FNH used. The 300blk one was still harder and did sound loud when it came loose whereas the 6.5DMR came loose so easily it didn’t even feel like it was torqued after the heat differential did its thing. The barrels were still cool to the touch and the flash hiders were hot but could be on my palm for a second or two before they burned so they weren’t glowing hot and were cooler than shooting even a mag through them to give you some idea. Be careful with adding too much heat to the barrel to avoid wrecking the heat treat and be careful applying too much torque as the barrel unscrewing from extension is a real risk. I angled the barrels level in the vise so that the ice and alignment rod plugs would allow water from the melting ice inside the barrel to slowly dribble up the front as it melted as the phase change from solid>liquid>gas of water takes a ton of energy and helps keep the barrel from overheating just like water cooled mgs in WWI. I didn’t even heat enough to boil the water in the barrels. I don’t have a torque spec but I wasn’t putting a ton of weight into the wrench on the 300blk and the 6.5 was super easy. It was maybe similar to an ar15 receiver extension torquing so I estimate around 50ft/lbs or less I was putting into wrench on 300blk but can’t be sure. If it didn’t work the first time, I would let barrel cool to ambient and then repeat the cooldown and heat up steps for a round two. I personally wouldn’t just keep heating it if the barrel is getting hot too. The heating process took about 2 min on each so was quite fast.
I’m hoping this helps someone else out there. Please report your experiences below to help others or suggest other ways. This was really daunting for me so tried to be as detailed as possible. Again, this is all still quite risky and entirely at your own risk with 0 guarantee it works as well for you but with my sample size of 2 it was perfect so please send it off to a competent smith if unsure or not successful after trying a time or two.
TLDR: cool muzzle end of barrel with ice inside barrel, apply heat to muzzle device at threaded part, apply moderate torque with aac brakeout tool with 12” cheater bar.
Cheers!