5.56 SPR Upper Build: 16" Rosco K9 Intermediate Gas Nitrided 416R Barrel + BCM Demo Upper + KAK 13" Handguard

[WeR0206]

The use case for this build is a low cost shorter length SPR made with some parts that I already had on hand except for the barrel and gas block.

Amazon list for tools/supplies mentioned in the videos/thread:
https://www.amazon.com/hz/wishlist/ls/3QMZQX5CVKWR2

Build videos:

1. AR-15 16” SPR Upper Build 5.56 (1/4): Taking Barrel Measurements and Checking Headspace
   
2. AR-15 16” SPR Upper Build 5.56 (2/4): Installing Dust Cover and Forward Assist
   
3. AR-15 16” SPR Upper Build 5.56 (3/4): Installing Barrel Into BCM Thermofit Upper
   
   
4. AR-15 16” SPR Upper Build 5.56 (4/4): Installing Gas Block, Muzzle Device, and Handguard

Build list (note re: pricing, some of these parts were purchased during Holiday sales):

• Upper receiver: BCM Demo Upper (# FLAT-TOP-UPPER-M4-DEMO) ($50) (thermofit)
• BCG: Radian Enhanced Nitride BCG (# R0081) ($179)
• Gas tube: Rosco Intermediate Nitrided Stainless Steel Gas Tube (comes with barrel)
• Gas block: Rosco .750” V2 Set Screw Gas Block (# ROS-NGB-002) ($38)
• Barrel: Rosco K9 16” 1:7 Intermediate 5.56 Nitrided 416R Barrel (# K9-16-HY-556-7-I) ($135)
• Handguard: KAK 13” Handguard (# MO-441-1010-006) ($110)
• Muzzle device: Cross Machine Tool Tactical Combat Flash Comp (QPQ Stainless) ($79)

Total cost: $591

Barrel inspection notes:

• Weight: 34.6 oz (2.2 lbs)
• Gas system length: Intermediate
• Gas port size: 0.088”
• Gas block journal: 0.7505”
• Barrel Extension: 0.992”
• Headspace gauge check (on fully stripped Radian bolt):
  • 1.4636”: closes
  • 1.4646”: doesn’t close
  • 1.4686”: doesn’t close
  • 1.4736”: doesn’t close

Build steps:

1. When the barrel arrives clean the chamber and bore with 91% Isopropyl alcohol then take measurements for weight, gas port size, gas block journal, and barrel extension
2. If you have a borescope, inspect the barrel and take pics/videos to document the initial state of the barrel. If you do see something like weird coloration, damage on the lands/grooves/crown/etc. still try shooting the barrel. If it doesn’t shoot and you think it’s from a documented defect you can always send the barrel back in to the manufacturer
3. Using a 1.4636” min headspace gauge and 1.4736” field gauge make sure the chamber is within min/max headspace specs by checking if a fully stripped bolt will close/twist on the gauges
4. If multiple stripped uppers are available find the one with the tightest fit on the barrel extension (I prefer thermofits)
5. Once the upper is selected (if needed) install the dust cover and forward assist
6. Clean the barrel nut threads, inside of upper, and barrel extension with some alcohol to remove any oil residue, etc. before installing the barrel into the upper
7. If building an accurized upper that’s not thermofit apply Loctite 620 to barrel extension (optional: apply a little aeroshell to the barrel nut threads to protect them from the loctite)
8. If thermofit, heat upper threads with heat gun on high for about 2 to 2.5 minutes then quickly slide the barrel into the upper making sure it fully seats. Wipe any excess loctite from the barrel nut threads and inside upper near barrel extension
9. Allow upper to cool (note the loctite needs 24 hours to cure before shooting)
10. Apply aeroshell 64 assembly grease to the barrel nut threads and shoulder of barrel extension
11. Season the barrel nut threads by tightening close to torque spec and loosening a few times
12. With upper secured internally by the lugs with Midwest Upper Receiver Rod, Magpul Bev Block, etc. torque barrel nut to handguard manufacturer’s specifications (most are around 50-60 ft-lbs)
13. Attach gas tube to gas block using Midwest Industries lower receiver block then attach gas block with cup head screws @ ~20 in-lbs (no threadlocker until after function check). Verify gas port alignment w/ scope. *After function check replace cup head screws with knurled tip screws & apply threadlocker if you wish
14. With upper secured internally via the lugs, oil the muzzle threads and attach muzzle device (torque to ~15 ft-lbs). Note if muzzle device will be used as a suppressor mount add a few drops of Rocksett to the muzzle threads before attaching muzzle device (let cure for 24 hours before use)
15. Attach handguard (follow manufacturers specs re: use of threadlockers, oil, etc. on handguard screws and torque specs) (look at Loctite 246 or Vibra-tite VC-3)
16. Insert BCG and charging handle
17. Attach upper to lower and do a full function check

Threadlockers and grease:

• Assembly grease (for preventing different metals from galvanizing together, used on barrel nut threads, etc.):
  • AeroShell 64 Grease ($10)
  • TW25B Gun Grease ($20)
• For gas block screws:
  • Rocksett or Loctite 246 (high temp/oil resistant)
• For bolt carrier gas key screws:
  • Loctite 246 (high temp/oil resistant) or Vibra-Tite 121
• For muzzle devices/suppressor mounts:
  • Rocksett
• For handguard screws, scope bases, etc.:
  • Vibra-Tite VC-3

 

[Cascade Hemi]

Single use knurled 10-32 set screws, torqued to spec, are a better solution than rockset (or equivalent).

 

[WeR0206]

Single use knurled 10-32 set screws, torqued to spec, are a better solution than rockset (or equivalent).

I agree. I use cup tip to do a function check and if everything works well I swap them out for knurled tip screws. Just edited the OP to specify this.

 

[flogxal]

Good point on degrease before threadlocker, any oil/grease tends to render the threadlocker useless. I've had fasteners come loose in other situations (mountain bike fasteners) where I'd missed some oily residue. Knew a guy who crewed in top fuel 1/4 mile who was obsessive about threadlocker prep with that big bomb of an engine!