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Discussion Starter #1
Hi there,

As I understand, several former eastern bloc countries made 5.56mm variants of their 5.45mm military rifles both to align to the NATO cartridge (and to satisfy export demands I guess). The AIMR SBR seem to be produced in almost identical versions for the two calibers.

It is self evident that the bolt head, barrel and mags would need to be changed in the production setup, but are there any other components that differ for the 5.56mm versions? Eg. Is the bolt carrier, trunnion etc. identical for 5.56mm and 5.45mm versions? In the event that other components than the ones mentioned above differ; does anyone know the reason for them being changed/adjusted?

Any insight would be appreciated, thanks!
 

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you don't even need to change bolt. 5.45 bolt will run well with 5.56. it is slightly off-spec but it will run. i actually had built 223 ak out of bulgy ak74. i changed nothing but the barrel (ak-builder bulgy 22mm journal 5.56 barrels)
feed lips on some of 5.56 mags (not all) are slightly higher than on 5.45 mags and therefore some small amount of metal in that area of the trunnion needs to be take out. overall there is very little of the changes needed. as matter of fact ALL thin stem carriers/bolts on all AKs from any country will be interchangeable between each other. same with the thick stem carriers/bolts.

also thick stem bolts front trunnions will not interchange with thin stem front trunnions.

AK-47/AKM thick stem bolt/carrier and 23mm front trunnion
AK-74/100 thin stem carrier/bolt and 22mm front trunnion

rear trunnions are interchangeable throughout although cosmetically will differ, but rivet locations are the same.

mag latches/trigger guards that have latch installed will not interchange very well but can be made to work. also cosmetically will have some variations between 47/AKM to 74/100
this only refers to stamped receiver AKs. milled receiver parts will interchange with slightly lesser regularity although major components still will be pretty swap-able.
 

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Discussion Starter #3
Thanks Mike, based on your input it may seem the AK design is more "AR-like" than I first believed.

Even if a 5.45 version could be made to work with just a barrel change, would it be correct to summarize that the following parts would have different part numbers in production from the same (5.56) product model?;
- Barrel
- Bolt head
- Mags
- Trunnion (depending on mag design)

I thought there were more changes than this for the dual caliber models (and also from country to country) but it seems global AK production is well organized. :)
 

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Thanks Mike, based on your input it may seem the AK design is more "AR-like" than I first believed.

Even if a 5.45 version could be made to work with just a barrel change, would it be correct to summarize that the following parts would have different part numbers in production from the same (5.56) product model?;
- Barrel
- Bolt head
- Mags
- Trunnion (depending on mag design)

I thought there were more changes than this for the dual caliber models (and also from country to country) but it seems global AK production is well organized. :)
sure. i'd say it is so.

also WARSAW pact required interchangeability of weapon parts and ammo. that is why many AKs parts are interchangeable. Pact members were not allowed to stray away from general design. few exceptions were made but in general everybody stayed on the same page if not approved otherwise. not until USSR started loosing it's grip on satellite states things began to change and evolve in AK world.
 

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The military would not use a 5.45 bolt to run 5.56 ammunition. That is a USA thing. The 5.56 magazines are quite different in dimensions by country as are the receiver mag wells and bolt types.
The Romanian for example Md86 in 5.45 uses a different gas block than the Md97 in 5.56. Also the bolt lengths are different as are the bolt carriers different.
Generally in 5.56 there is quite wide variation in the magazines, barrel profile and other features for AK's. Much less commonality of parts than in 7.62x39. The countries that adopted 5.45 had somewhat commonality of parts but also a number of deviations from a standard pattern.

In the USA there is a common barrel swap available in the Bulgarian AK74 pattern rifle to convert it to fire 5.56. The Bulgarian 5.45 bolt and eceiver type will reliably feed the 5.56 round with a Bulgarian waffle 5.56 magazine.
Certain 5.45 magazines can be made to fire 5.56 rounds but generally requires a different follower and sometimes a magazine mod to load a full 30 rounds.
 

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Discussion Starter #6
The Romanian for example Md86 in 5.45 uses a different gas block than the Md97 in 5.56. Also the bolt lengths are different as are the bolt carriers different.
This is interesting. Generally; introducing design changes in production is expensive and is only done if there is a pressing reason to do so.

Both military calibers seem to generate a maximum pressure of 380MPa. What is the reason for introducing a different gas block in production for 5.56 if the pressure is the same for both calibers? I mean, the absolute minimum bolt carrier travel is only be 6mm longer for 5.56 than for 5.45. Is different gas pressure required in order to increase travel margins correspondingly?

It would also be interesting to understand the differences in design features of the bolt 5.56 carrier. When you say longer for 5.56; is it due to the gas piston being lengthed 6mm in order to compensate for the 6mm longer travel mentioned above, keeping the rest identical, or is the longer bolt carrier different in other ways?

Any insight and thoughts would be appreciated!
 

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This is interesting. Generally; introducing design changes in production is expensive and is only done if there is a pressing reason to do so.

Both military calibers seem to generate a maximum pressure of 380MPa. What is the reason for introducing a different gas block in production for 5.56 if the pressure is the same for both calibers? I mean, the absolute minimum bolt carrier travel is only be 6mm longer for 5.56 than for 5.45. Is different gas pressure required in order to increase travel margins correspondingly?

It would also be interesting to understand the differences in design features of the bolt 5.56 carrier. When you say longer for 5.56; is it due to the gas piston being lengthed 6mm in order to compensate for the 6mm longer travel mentioned above, keeping the rest identical, or is the longer bolt carrier different in other ways?

Any insight and thoughts would be appreciated!
The gas port in the barrel was changed when the AK74 was introduced from the 7.62 port of 45 degrees to a 90 degree port. The gas block (USSR) was then changed to 90 depress vs the 45 degree. The Romanians stayed with the 45 degree block but used the 90 barrel port. Allegedly to avoid bullet jacket shear that was experienced on the 45 degree ovoid gas port opening in the barrel.

Various countries adopted different bolt types for the 5.56 caliber., short or long lugs and large or small stem.
I believe the bolt change and carrier change were made to accommodate the longer 45mm 5.56 cartridge vs the 39mm 5.45.

By the time the 5.56 production was happening the USSR was virtually or already defunct.

allessonnogwat might be along to offer more detail.
 

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Discussion Starter #8
So if I got this correct; the reason for the Md86/Md97 gas block variations is not tied to specific requirements for the different calibres, but rather a reliability upgrade that was due based on jacket shear experiences with the 5.45 gas block.

The length increase based on increased bolt carrier travel seems reasonable.

Do you know if the AIMR have different bolt lug lengths and stem dimentions for the two versions?
 

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So if I got this correct; the reason for the Md86/Md97 gas block variations is not tied to specific requirements for the different calibres, but rather a reliability upgrade that was due based on jacket shear experiences with the 5.45 gas block.

The length increase based on increased bolt carrier travel seems reasonable.

Do you know if the AIMR have different bolt lug lengths and stem dimentions for the two versions?
The gas block was not the issue but rather the barrel gas port opening. But almost simultaneously they changed to the 90 degree block as well as the port change. Except the Romanians. AIMR and Md86 same operating system in 5.45. AFAIK.
 
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