Metric values:
Classification: SL8-1, SL8-4, SL8-5, SL8-6
510 mm Factory Barrel - Non-Restricted
480 mm G36E Barrel - Non-Restricted
318 mm G36k Barrel - Restricted
228 mm G36c Barrel – Restricted
420 mm HK243 Barrel - Restricted (North Sylvia has no interest in importing HK243 barrels at this time. Sad!)
Dimensions (minimal lengths for legal compliance):
SL8:
980 mm (no spacers)
G36:
758 mm stock folded
G36K:
615 mm stock folded
G36C:
500 mm stock folded
HK243:
667 mm stock folded (of interest if using HK243 barrel on SL8)
Trigger Pull:
SL8:
16.28 N / 3 lb 9.2 oz
G36/HK243:
37.66 N / 8 lb 7.4 oz
Barrel Torque Spec:
90 N m / 66 ft-lbs.
Sight Rail Torque:
0.015 N m / 22.5 inch pounds
Regarding the trigger pull, there exists a way to use AR trigger groups, including match triggers (and AR hand grips):
Image is from sl-8 . de
The HK G36 FBI lower needs a hole drilled through the housing to accommodate for the extra parts of the SL8 trigger group. There is an indent/mold mark in the housing exactly where the hole needs to be drilled. Use a centre punch to guide your drill bit. Use drill bit size 0.157" or 5/32"
On the top of the receiver there are 4 pads, they need to be sanded down to fit. Amount various, but around 1/16 of an inch. Test fit the lower, if it doesn't fit, evenly sand down the top pads a little, test again. Repeat. DO NOT RUSH.
These extra parts are supposedly the hammer disconnecter (supposedly it reduces the risk of a slam-fire, but the SL8 doesn't have an auto-sear, nor the interface on the bolt carrier for the auto-sear) and not having it allegedly can result in a reduced trigger pull. If you use the heavier trigger springs from the G36 instead of the SL8 strings, the trigger pull will be heavier. This step of adding the hammer disconnecter is optional, depending on how clumsy you are. Very clumsy: disconnecter + heavy G36 springs. Ballerina: no disconnecter and light SL8 springs.
You are responsible for every projectile that leaves your firearm.
Lenght-of-Pull
Also can include this info including images:
The stock-block conversion renders the length-of-pull more appropriate for a Dinka warrior or a Dutchman due to the longer receiver on the SL8 as compared to the HK243 or G36:
(Image showing a "made in bedroom, just like you!" stock block conversion aligned with G36 receiver with G36 stock installed. Alignment is at the lower front push-pin hole and comparison point is the stock hinge. Approximately 20 mm of extra length are evident on the stock block conversion.)
The original G36 stock was also quite long, and in a sense even HK "admitted" that with the IDZ replacement stock:
260 mm original (not G36C length) vs. 235 mm (convex IDZ stock) / 230 mm (concave IDZ stock) (fully extended):
(Image showing an original G36 stock aligned with a convex IDZ stock. A ruler is placed on top, with "0" aligned to the stock hinge. The fully extended IDZ stock measures to approximately 235 mm, while the original G36 stock measures to approximately 260 mm.)
Sadly, using a stock-block conversion severely limits the capability of SL8 to eject with the stock folded inwards.
There are at least two people in Canada who can do the conversion, and at least one of them has performed the conversion on several SL8s. Finding them (should they want to be found) however is another story. There are also at least three "desporterized" (or is it "ergonomified"?) SL8's in Canada, which again, you would need to find and encourage their owners to sell them.
Muzzle devices in 15 x 1R:
Can use G3 muzzle device. Can also use B+T APC223 and HK Mr. 223 / Mr. 308 but will require a crush washer as they do not have the spring to engage the splines. Crush washer sits inside the muzzle device and presses against the crown of the muzzle.
I cant get my barrel off, even with the proper tooling!
Some newer SL8's have locklite on the barrel nut. You need a special tool to "heat" the nut, without melting the reciever. Looks like a .223 headspace gauge or dummy round on the end of a soldering iron. Never seen for sale. Would need to create yourself or contact gunsmith.
Alternative: Take of handguard and put on welding gloves and load 100 rounds into magazines. If you don't have 20 magazines, then get a friend/neighbour/international student/Deliveroo driver to stand by with you and load your mags as they become empty. Maybe load magazines before putting on welding gloves, might be easier that way? Try both methods, see which is better, then post here to let us know.
Anyway, prepare the proper tooling and fire off 100 rounds in quick succession (follow proper precautions like not shooting at anything you don't want to destroy and wearing safety equipment and whatever).
Make sure firearm is unloaded and quickly disassemble rifle, including taking out bolt carrier, removing lower, piston, and piston rod. Maybe practice first? This should take no longer than 40 seconds. Oh, right, there are screws on the SL8 to remove the lower. Yeah, get the magazine loader to help and have several hex key wrenches. Practice first before spending 30$ heating your barrel!
Check that the firearm is unloaded and there is no cartridge stuck in the chamber.
By this point there should still be enough heat to have softened the locktite. If you are not able to complete this on the first try, get everything back together again and fire off another 100 rounds and try again.
Note that the difference between the melting points of the loctite and the nylon the receiver is made of
may be less than 20 ºC, or may be as far apart as 150 ºC. If someone with a lab wants to test a sample of SL8 for the melting point, that would be appreciated.
Always use the mandrel! (And line the inside of your wrench with felt to prevent scratching the barrel.)
I want to repair the body, handguard, stock, or any other Nylon part of the rifle, what is the best way?
Depending how much you need to fix, you could use Devcon Plastic Welder II, one of the only glues on the market that is "active" with Nylon 66.
Another option is to use a plastic welder. For material, its best to get a scrap piece of HK nylon, like an old handguard, or factory -4/-5 top rail. Something that holds little value, but is made with HKs nylon 66 + 30% glass fiber mixture.
Yeah, don't bother trying to use Devcon II
for structural repairs, it is complete and utter rubbish and does not stand to vibration, making trying to use it a waste of time. One shot and it cracks. Welding is the only way to go
for structural repairs.
Barrel-related Info:
There are several barrel lengths as written in the first post of this document, but here we will get into more technical detail.
Every barrel is interchangeable between the SL8, G36, and HK243, with the only difficulty being to remove the barrel nut the first time on newer production rifles as the lock-tite needs to be heated up to near the melting point of the nylon-66.
No headspacing is required after a barrel has been changed out as distances are kept correct due to the design of the trunnion and precise manufacturing tolerances, provided that the barrel nut is torqued correctly to 90 N m.
The chamber of the SL8 will not fit Manson Precision Reamers 5.56 GO/NO GO gauges for whatever reason with the barrel mounted (multiple barrels were tested). With the barrel removed, the chamber end and the bolt mate almost flush (unable to measure reliably the gap).
The barrel of the SL8 and the G36 (and likely the HK243 too) are allegedly both made on the same mandrel, with the G36 barrel being profiled to be lighter/shorter, so this is likely not due to the SL8 chamber being “sporting” and thus made to tighter tolerances. Usually, a military barrel will be out of spec with a slightly bigger chamber than sporting specification minimums allow for to accommodate a vast variety of ammunition be it corroded, damaged, poorly manufactured, or dirty.
Don’t shoot corroded or damaged or dirty ammunition.
SL8 throat length is sufficient to accommodate an over-all-length of 65.7 mm / 2.59 inch.
Gas blocks:
There are five gas blocks, of which four are known. If you have an HK243 gas block, then please add the info or submit it for analysis:
Clockwise from top left: the SL8 gas block, the G36 old style, the G36C, and the G36 new style
The four known gas blocks are (clockwise from top left) the SL8 gas block, the G36
old style, the G36C, and the G36
new style.
The differences are as follows:
SL8 gas block is in upper left corner and the bigger diameter and width of it can be easily seen.
SL8 gas block has an inner diameter of 20 mm (and an external width of 26 mm, so it will not fit in the HK243/new G36 handguard, nor will it fit in a handguard with a heatshield in it), while all the G36 gas blocks have an internal diameter of 16 mm (and an external width of 21 mm).
The G36C gas block has external truncations on the bottom end of the cylinder, trimmed at 5 degrees to allow for easier egress of the piston rod.
The G36
old style (and the SL8 gas block & G36C gas block) has a linear length of 23.5 mm along the barrel, while the G36
new style block has a linear length of 37.5 mm along the barrel.
Detail of the G36 new style and the G36C gas blocks.
Gas port hole diameter on the barrel is:
• 1.65 mm for the 480 mm/510 mm barrel (SL8, G36)
• 1.73 mm for the 318 mm barrel (G36K)
• 2.01 mm for the 228 mm barrel (G36C)
Gas port diameter for the HK243 is unknown at present, please add if possible.
Gas pistons:
There are two gas pistons:
Full size piston is on top, the "K length" piston is on the bottom.
Full size:
• 51 mm over-all-length
• 14 mm depth for piston rod.
• used on the 480 mm/510 mm barrels (SL8, G36, HK243, HK 416, HK417, MR223, MR308).
“K length”:
• 41 mm over-all-length
• 8 mm depth for piston rod.
• used on the 318 mm and 228 mm barrels (G36K, G36C).
Gas pistons have three piston rings and diameter of 10 mm. The diameter of the hole for the piston rod is 6 mm.
The piston rings need replacing if the piston slides out of the gas block under the force of gravity when the rifle is held vertically.
Ensure that the rifle is unloaded before testing, and don’t forget to remove the piston rod. Alignment of piston rings does not matter as entropically they do not align under use.
Piston rods:
There are three lengths of piston rod:
From top to bottom: Full length, "K length", and "C length" piston rods.
Full size:
• 219 mm over-all-length
• 45.5 mm distance from plate to bolt carrier
• used on the 480 mm/510 mm barrels (SL8, G36, HK243).
“K length”:
• 176 mm over-all-length
• 42 mm distance from plate to bolt carrier
• used on the 318 mm barrels (G36K).
“C length”:
• 141 mm over-all-length
• 42 mm distance from plate to bolt carrier
• used on 228 mm barrels (G36C).
Piston rod diameter is 5 mm.
To modify full size gas piston to “K length”:
1. Drill the hole that the piston rod fits into 9.5 mm deeper into the gas piston. The diameter of this hole is 6 mm.
2. Cut 15.5 mm off the end of the gas piston that is opposite “the nipple”. Cut right up against the flange, but
do not cut the flange off.
3. Neaten things up, chamfer the entrance to the hole a little, remove burrs, chrome the interior of the piston-rod hole, clean your workspace, etc.
4. Done.
To modify full size piston rod to “K length”:
1. Trim 3.5 mm off the end of the rod closes to the metal plate. This is the end that strikes the bolt carrier.
2. Trim 38.5 mm off the other end of the rod. This will be at the inward side of the middle ridge.
3. Neaten things up, chamfer the cut ends a little, remove burrs, re-blue the rod or otherwise refinish it, clean your workspace, etc.
4. Done.
Rods and their associated pistons. From top to bottom: for SL8/G36/HK243, for G36K, and for G36C.
Note: It is not possible to modify the full-size, nor the “K length” rod to the “C length”.
Q: Help! My piston rod is bent and the firearm won't reload itself any longer! Why did this happen and what can I do to prevent it?
A: Your gas port is clogged up with carbon, bad news, and delayed maintenance. Replace the rod, clean the port, learn your lesson.
The HK243 hand guard
The HK243 hand guard is the new look of HK. This style of hand guard is also being used on the HK416 family of rifles and features the HKey mounting system for accessories. The HKey mounting system is not compatible with KeyMod as much as they look similar. It is the opinion of this author that both these systems are rubbish and that M-Lok’s camming bolts are the superior option. But what do I know? I’m just a biologist.
The upside of this hand guard is that it is narrow and fits the hand nicely, while the downsides are that it weights about half a kilo and has sharp edges. The hand guard mounts using a screw to secure it instead of the push-pins that the polymer hand guard uses, which can also be seen as a downside (because disassembly now requires tools) or an upside (because now it is much more difficult for the user to stash drugs in the hand guard under the heat shield) depending on whom you ask. It is the opinion of the author that this change is a vote of confidence of HK’s engineers in the gas system not requiring access outside of regular service. Oiling the gas piston leads to the oil burning off pretty quickly and doesn’t seem to add or take away anything for functionality and reliability, so maybe HK has a point: the gas system doesn’t need to be touched.
The HK243 hand guard is a couple of millimeters too narrow to let the SL8 gas block pass through it. So to get it to work on the SL8 there are two approaches to take. One is to widen the hand guard.
Widening the hand guard would require cutting or grinding or scraping out material along the length of it from the inside, thinning the walls to make a wider passage for the gas block. This will likely be only possible for someone with an EDM machine.
You may be tempted to just spread the walls of the hand guard a bit with a lever or a jack or some hydraulic doo-dad but don’t do that because the aluminium will fracture and the hand guard will be rubbish.
Speaking of rubbish, the HK243 hand guard is made of aluminium with steel inserts and steel nuts. It is 99.95% recyclable. At it’s end of life you can take out the three screws on each side holding the side panel and the two screws on the receiver end that hold the tab that slots into the receiver, and throw everything into the recycling, where it will then hopefully be made into HK416 receivers. Or something like that. Be responsible and engage in waste diversion!
To get the HK243 hand guard to fit on your SL8, the other approach is to thin the gas block out to make it pass through the hand guard:
• Step 1:
Make sure the firearm is unloaded.
• Step 2: Take off the hand guard you have how, remove the piston rod and the piston.
Pin removed.
• Step 3: Knock out the roll pin on the gas block. The pin has a diameter of 6 mm and can be pushed out in either direction. If you chew up your pin then at least put a drop of oil on it to prevent corrosion.
• Step 4: Slide the gas block off the barrel.
Make sure that your measurements are centered, and not skewed as in this image.
• Step 5: Fish out some calipers and set them to 10 mm. Eye-ball the centre-line of the gas block and mark 5 mm to the top and 5 mm to the bottom of the centre-line. Or eyeball both the lines and just run your pencil along the lips of the caliper.
In the image above, the eyeballing of the centre-line is being done from the ends of the gas block because the centreline has to be horizontal with the gas block so that the sides are shaved off to perfect vertical. Also in the image above the calipers are not centered.
Tape laid down to mask the work area.
• Step 6: Using some tape, mask off the rest of the gas block except for a hopefully centered band 10 mm tall. Join up the marks you made on both ends of the gas block. If the tape doesn’t stick then likely there is oil on the gas block. Painter’s tape should be sufficient.
• Step 7: Once you have masked your work area, get out a flat file and file down everything between the lines so that the unmasked surfaces are flat instead of curved. Only file the sides, and do not get carried away.
The filing is done.
• Step 8: Test that the gas block passes through the hand guard. It should. You can polish the surfaces to be mirror smooth, or you can grit-blast the gas block and reblue it (do not blast the interior where it fits against the barrel nor the piston chamber) or nitride it or whatever. If you are not refinishing and are just leaving it as is on the image then rub a bit of oil on the surfaces to prevent corrosion.
• Step 9: Put everything back together again.
Bonus: How to change the screw to a push-pin (there is no going back so chose wisely!)
Screws undone.
• Step A: Undo the three screws on the right side of the hand guard (the side with the threads).
The pin passes through where the threads once were.
• Step B: File out the threads until the push-pin can pass through. Or drill out the threads.
Put a pin in it!
• Step C: Put the hand guard back together again.
The pin will make the hand guard not need tools to remove, but it may still be a very tight fit on the receiver, and you may find that you need to use a screwdriver to carefully pry the hand guard forwards in order to take it off.
Barrel profiles
Barrel profile data is released under Creative Commons Non-Commercial Share-Alike 4.0 International (CC NC-SA 4.0)
You can profile barrel blanks using this info and sell them, you just can't sell these profiles themselves as data. These profiles are for everyone to use without profit seeking.
You can find the 480 mm barrel profile here as
.igs,
.ipt,
.dwg, and
.pdf.
This info is based on measurements taken from a barrel and some calculated values. It is not from HK databases or official HK data, so there could be some differences. There also might be some mistakes, so check seven times, and run the lathe only twice. Or whatever. Also, this profile is based on the assumption that you are using a G36
new style gas block, and assumes that you want to install the lug and are going to be using an HK threaded muzzle device.
• If you want to have a different muzzle device then change the threading of the first 10mm.
• If you want to use a G36
old style gas block, then go ahead and use it with no changes required.
• If you don't want the lug then don't cut the slot for the roll pin that is closest to the muzzle.
• If you want to use an SL8 gas block, then your barrel should have 20mm x 26mm spot for it so change the profile accordingly.
• If you want to save money and not get the splines done at the muzzle end and instead use a crush washer, then ignore the splines.
• If you want to change the . . .
You know what? Just talk to your machinist, ok? And don't tell them you know better, because you don't, unless you are a machinist and an engineer.
