105mm or 106mm?


#1

On another forum there is a discussion about US recoilless rifles and their calibers. This is not my area of expertise, nor do I have any reference material that is helpful, but I am interested in figuring this out.

Apparently the M-27 RR from the ealry 1950’s is 105mm in caliber. It’s replacement, the M-40 from the 1960’s is 106mm in caliber. Information from various posters on this other forum, including quotes from Wikkipedia, claim that the 105mm ammunition can be fired in the 106mm, but not the other way around. Others have suggested that the 106mm is actually a 105mm but is called 106mm to avoid confusion.

Are you confused yet?

Does anyone have information to help sort this out?

Up to this time, I’d only heard of the 106mm RR…

AKMS


#2

AKMS, The 105mm M27 as I read somewhere was used by the National Guard and I have seen such ammo being disposed of, unfortunately only after the job was done and it confused me because the shell I have seen was all orange and looked like a HESH.

The US recoilless rifles/guns I know of are:
57mm M18
75mm M20
90mm M67
105mm M27 (the one in question here)
105mm T9 howitzer
106mm M40
120mm M28 (Davy Crockett, nuclear)
155mm M29 (Davy Crockett, nuclear)

Here something on the rifle and also on the ammo. The caliber issue of 105mm vs 106mm is explained somewhat different:
globalsecurity.org/military/ … 27rclr.htm

Here images of the 105mm M27:
warwheels.net/images/m38a1cRRjeepHAUGH2.jpg
warwheels.net/images/m38a1cRRjeepHAUGH1.jpg
military-info.com/book/N08_R105/Bk_r105.htm

Yes, I knoiw I did not really answer the question about the 105mm ammo.
Has anyone a manual or good images of this ammo?


#3

Having 2 105mm recoilless rifles M27 and M40 using small different ammo at the same time would be very confusing.
The M40 rifle actually has a bore diameter of 105mm, but is called 106mm for logistical reasons.
This and the complete development can be read in Engineering design recoilless rifle, 1976.

You are looking for a reason why 106mm can’t be fired from a M27 rifle?

The cartridge cases are much alike only for a more bottlenecked M32 case for the M27 rifle.

Pictured is a brown lackred 105mm M32 case beside a 106 T53E1 wich is made from an old M32 case.

As you can see the perforation holes are in one line at both case.
Beside them is a normal 106mm made M93B1 case. Note the holes are not in line and the case is welded.





#4

For downloading the Engieering design handbook:
dtic.mil/cgi-bin/GetTRDoc?AD … tTRDoc.pdf


#5

Western, thanks a lot for your info. The HESH (HEP-T) shell I have seen was for your M32 case.


#6

Do each of those holes have to line up with another hole in the breach or does it work a different way?

What is the off-centre plug on these cases for?


#7

Thanks for all of the information. So it appears that the 105mm rounds from the M-27 can not be used in the 106mm M-40 due to the bottleneck design of the 105mm case, even though both of the rounds are actually 105mm.

AKMS


#8

Falcon,

The breach is in contact with the case only for the mouth and base.
The holes are filling plugs for finstabilized projectiles. The projectiles are placed first then the propellant is brought in.


#9

Thanks for the explanation. Even with the holes there, you would think the case would bulge out a bit. However, it obviously didn’t, or the weapon would never have worked.


#10

I used to have a really nice finned projectile for the 105-mm RR and it was actually labeled 105-mm / 106-mm on its driving band. I’ll look for a picture. I think it was a HEAT projectile and not a HESH projectile.

Jason


#11

Jason, with fins it was a HEAT shell since it was desired to reduce the spin and stabilization had to be provided by the fins.
Same as with the HEAT shells on normal tank guns where the driving band is rotating free and is not fixed to the shell’s body where it is just used as a gas seal.


#12

Thanks EOD!!! I am going to try hunting down a pic of it :)

Jason


#13

Thanks for the help Western and EOD I always wondered the difference now I just need to track down a 105mm RR casing


#14

[quote=“Western”]Falcon,

The breach is in contact with the case only for the mouth and base.
The holes are filling plugs for finstabilized projectiles. The projectiles are placed first then the propellant is brought in.

[/quote]
Western, I’m puzzled by your explanation. The holes in the side of the case are to let the propellant gas into the chamber so that most of it can be diverted through the rear venturi exhaust, thereby balancing the recoil of the projectile travelling down the barrel.

I have a 57mm recoilless round which is made on the same principle, and that has pre-engraved rifling bands on the projectile - it is nothing to do with fin stabilisation.


#15

Tony,

I think you mixed 2 answers on falcon’s questions.
57mm, 75mm, 105mm, 106mm rifles all work with the same principle.
There is a space around the perforations where pressure is rising and escapes through the venturi.
That answers the first question.

On the second I shouldn’t use the word "holes"
The fillingplug is used because the long fins of the HEAT don’t allow normal assembling. This has nothing to do with the perforations, the same fillingplugs you can find on other casings with HEAT projectiles.


#16

[quote=“Western”]Tony,

I think you mixed 2 answers on falcon’s questions.
57mm, 75mm, 105mm, 106mm rifles all work with the same principle.
There is a space around the perforations where pressure is rising and escapes through the venturi.
That answers the first question.

On the second I shouldn’t use the word "holes"
The fillingplug is used because the long fins of the HEAT don’t allow normal assembling. This has nothing to do with the perforations, the same fillingplugs you can find on other casings with HEAT projectiles.
[/quote]
Right - understood, thanks.