I used 2'' spacers off the top of the receiver sides to space a 4' level and clamped it together to take my measurements. I felt if any exterior parts of the receiver would be square to each other it would be where the bolt rides. However a flat edge check of these surfaces reveals less than perfect surface flatness. I feel a truly accurate check of barrel trueness would base off of the machined internal rails. That all said here is my data, all rifles are folders, in consecutive serial numbers.
With the level clamped as above I took a total of 4 measurements from it; each side of barrel from muzzle to straight edge and from the barrel under gas tube. All measurements taken with receiver field stripped.
Update Dec.12
I have gone over the rifles again and attempted to measure barrel alignment to receiver by another method. This time I took a brand new one piece stainless cleaning rod and inserted it down the barrel of each gun and performed a visual check against the machined internal receiver rails. All guns passed this check, the rod was parallel to the internal rails in all cases. When attempting to take measurements I got some interesting results that I will specify by rifle.
With regards to "bent" receivers and flatness of surfaces, I must make some clarifications and corrections of some of my previous comments. My initial flat edge check of the receiver walls and subsequent judgement appears to have been made in haste. Using a 6" machinist rule as a flat edge against the external receiver walls shows surprising flatness rearward of the trunnion and riveting for such. A check of the internal machined rails showed PERFECT flatness. One rifle had a rail that depending on how I held the rule may have felt a bit off, but that is me getting very picky, and I can be OCD about things. All rifles pass inspection in this regard.
Misaligned gas blocks/front sight and condition of riveting:
Two of the rifles have near bang on centre gas blocks, the other two are slightly to the right but not nowhere near as bad as one example someone showed a picture of. Riveting is satisfactory and more than solid. Cosmetically one or two rivets on a rifle may have a slight angled look to the head but that is it. One rivet head had a double strike imprint. I have seen journey metal fabricators do much worse.
Rifle 1: measurement at muzzle was 2.5" from the left and 2.063" from the right. About a 3/16th difference in measurements showing bias (bend) to the RIGHT. To centre would require a 3/32nd shift to left based on this measurement at the MUZZLE. To be precise the edge of the grenade launching attachment.
Measurement at barrel under gas tube, behind launcher was 2.5"/2.25" for 1/4 inch difference, to centre using these numbers would require a 1/8th shift to the left.
Based off these numbers it would seem obvious that the barrel is skewed to the right. However a quick and dirty string check would show it to be relatively straight visually. The front sight post on this rifle is also in the centre, the best of all the rifles. Grip has slight shift, need to tighten grip bolt.
This rifle is the most out of spec by my measurements.
Rifle 1 Update: Folding stock is straight to rifle, slight cant to right when viewed from rear. Acceptable, I can tweak it if it bugs me enough.
Measuring from the cleaning rod out to internal rails: rod is almost centred between the rails, might be slightly to left but still parallel to the rails.
With the level clamped as above I took a total of 4 measurements from it; each side of barrel from muzzle to straight edge and from the barrel under gas tube. All measurements taken with receiver field stripped.
Update Dec.12
I have gone over the rifles again and attempted to measure barrel alignment to receiver by another method. This time I took a brand new one piece stainless cleaning rod and inserted it down the barrel of each gun and performed a visual check against the machined internal receiver rails. All guns passed this check, the rod was parallel to the internal rails in all cases. When attempting to take measurements I got some interesting results that I will specify by rifle.
With regards to "bent" receivers and flatness of surfaces, I must make some clarifications and corrections of some of my previous comments. My initial flat edge check of the receiver walls and subsequent judgement appears to have been made in haste. Using a 6" machinist rule as a flat edge against the external receiver walls shows surprising flatness rearward of the trunnion and riveting for such. A check of the internal machined rails showed PERFECT flatness. One rifle had a rail that depending on how I held the rule may have felt a bit off, but that is me getting very picky, and I can be OCD about things. All rifles pass inspection in this regard.
Misaligned gas blocks/front sight and condition of riveting:
Two of the rifles have near bang on centre gas blocks, the other two are slightly to the right but not nowhere near as bad as one example someone showed a picture of. Riveting is satisfactory and more than solid. Cosmetically one or two rivets on a rifle may have a slight angled look to the head but that is it. One rivet head had a double strike imprint. I have seen journey metal fabricators do much worse.
Rifle 1: measurement at muzzle was 2.5" from the left and 2.063" from the right. About a 3/16th difference in measurements showing bias (bend) to the RIGHT. To centre would require a 3/32nd shift to left based on this measurement at the MUZZLE. To be precise the edge of the grenade launching attachment.
Measurement at barrel under gas tube, behind launcher was 2.5"/2.25" for 1/4 inch difference, to centre using these numbers would require a 1/8th shift to the left.
Based off these numbers it would seem obvious that the barrel is skewed to the right. However a quick and dirty string check would show it to be relatively straight visually. The front sight post on this rifle is also in the centre, the best of all the rifles. Grip has slight shift, need to tighten grip bolt.
This rifle is the most out of spec by my measurements.
Rifle 1 Update: Folding stock is straight to rifle, slight cant to right when viewed from rear. Acceptable, I can tweak it if it bugs me enough.
Measuring from the cleaning rod out to internal rails: rod is almost centred between the rails, might be slightly to left but still parallel to the rails.
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