5.5mm

Question
Thank you for that response. can you tell me what size shell casing it would take the chamber looks like it will take a large bottle neck shaped casing maybe with the 22 bullet in the end. It is extensively engraved has has an engraved scope mount.It holds 4 shells under the butt has 4 holes for ammo with an engraved cover.it looks more like a fancy hunting gun it has a stag engraved on it also sorry i left out this info.
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-----Question-----I have a rifle marked 5.5MM from some of the codes i thick it was made in
Austria in 1936.some German proofs crown over U crown over B. It is nicely
engraved I would just like to know what caliber it really is any help would be
appreciated. PS The 5.5mm is over the number 52 but no line between them. Do you
think possible a 6x52 round? as i know some 9.3mm guns are marked 8.8 and some 8
mm guns are marked on the gun 7.7mm There are other markings Suhl circled I
think that is a town in Germany Also K.MG I think that means copper jacketed
bullet. AND N 5gr i figure that to be smokeless powder but the 5gr? 5gr bullet?
5gr powder? Thank you. The gun is also marked          R.MAHRHOLDT IN INNSBRUCK any info would be appreciated
-----Answer-----
By what you have explained, I think you have a 1930's era  German Training rifle that shot the 5.5mm (.22 cal) round. The n 5gr stands for 5 grams of Gewehr Blattchen Pulver
(military flake powder). The KMG stands for "Copper Jacketed Lead bullet.

Depending on condition, I would estimated the value at around $300.00. I hope this helps you understand the gun a little better.

vic

Below is an article from Steve Meyer, German Gun Collectors Association.

A Proof Mark Primer, 1891-1939
By Steve Meyer, German Gun Collectors Association

Unravelling the history of a German sporting gun or rifle is often a difficult proposition. With the exception of a few larger makers like J.P.Sauer, Merkel and Krieghoff most records are lost and only the guns themselves bear testimony to the history, workmanship and artistry of German gunsmiths.
Fortunately, one source remains to us, and that is the marks on the guns themselves. Beginning with the first German proof law that came in to effect in the early 1890s, all sporting firearms produced in Germany were required to pass rigorous strength and safety tests at a government regulated proof house. Over the years German proof laws, like those of other European countries, were revised to cover changes in gunpowder, materials and design, and changes in proof law and the marks prescribed can be very helpful in tracing date of manufacture, chambering and originality.
I won't attempt to cover the all ins and outs of German proof law here in detail, but there are a few bits useful for dating that frequently come in handy.
Guns and rifles built in Germany and the German principalities before the 1890s typically did not carry proof marks, but the 1891 proof law 'grandfathered' these arms by mandating that they be submitted to the proof house and marked with a crown-over-V, the V standing for vorrat or 'on hand'. Any German gun or Rifle carrying a crown-over-V dates to 1891 or earlier.
Marks showing crowns over the letters B, U, (proof approved); G (rifled barrel), S (smoothbore) and W (choked barrel) span the period from the first German proof law until 1939, but there are other marks usually present that can narrow the date range greatly.
Rifles marked with only an indication of powder charge and bullet type as shown below date to 1912 or before:
2,67 g GBP
 St m G
These marks show 2,67 grams of Gewehr Bl鋞tchen Pulver (military flake powder) and Stahl-mantel Geschoss (steel jacketed bullet). Other possible types are "K m G" for Kupfer-mantel Geschoss (copper jacket) and "Bl G" for Blei Geschoss (plain lead bullet).
After 1912 marks were changed to show just bullet type and bullet weight as in:
St m G
12 g
usually accompanied with a crown-over-N or the word Nitro, specifying smokeless powder.
Shotgun barrels of this era will sometimes be marked with a powder charge over a shot weight, but the style of the marks didn't change markedly over time. Note, however, that some will be marked with the high-pressure proof load, while others will be marked with the standard service load. Shotgun barrels will also be marked with the gauge designation in a circle and, unless they are specifically stamped otherwise, will be chambered for 65mm or 2-1/2" shells. As an interesting sidelight, I have seen more than one German double rifle advertised that bore crown-over-W marks on the underside of the barrels. A closer inspection revealed that their rifled barrels had been sleeved into the shotgun barrels or block and they were in no way original, or perhaps even safe. Caveat emptor!
The most frustrating but perhaps the most useful marks of the 1891-1939 era are the various sets of just numbers often present. The two most common sets of numbers applied were bore dimensions and actual proof dates.
Rifles from later in this era are sometimes stamped with a bore (not groove) diameter in milimeters, the most common being 8,8mm for one of the 9,3mm cartridges and 7,7mm for one of the many 8mm cartridges.
Many earlier rifles will be found with numbers indicating the "gauge" diameter of the bore (not groove). For instance 108/49 would indicate a bore diameter from .350-.359" and hence likely to be a 9,3mm chambering of some kind, while 172/28 (.300-.309") would indicate an 8mm of some kind.
See table below for rifle gauge numbers.
The other set of numeric marks often found on both rifles and guns is the actual date code of proof. The style of these numbers can easily be confused with the gauge markings described above, so be aware that both possibilities exist. Typical date codes will be three or four digits signifying month and year, as in 513 for May, 1913, or 1127 for November of 1927. Sometimes there will be a dot or slash between the month and year, but most often they are just run together. Often the date code is above a line, with another number below the line. The number below the line is that gun's actual entry number in the proof house records for that month and year.
All of the preceeding notwistanding, there is a great deal of variation in proof marks of German guns and entire books have been written on the subject of proofs. One of the best is Gerhard Wirnsberger's The Standard Directory of Proof Marks which is available from our bookshop.
With so little information yet discovered about so many German makers, a working knowledge of proof marks can be your guide to the hidden histories of German sporting arms.

Rifle Gauge Table
A brief table that is pretty helpful for pre-WWI German rifles shows the use of fractional 'gauge' numbers for barrel diameters. This was largest plug gauge diameter that would fit in the bore. The British proof law of 1868 had set up standard gauge sizes from .300" (172.28 gauge) to .450" (51.05 gauge) in .010" increments in the same sense that a shotgun bore is gauged, by the number of round lead balls of that diameter that make a pound. The Germans adopted this somewhat awkward system as well.
Gauge - inch
51.05 - .450
54.61 - .440
58.50 - .430
62.78 - .420
67.49 - .410
72.68 - .400
78.41 - .390
84.77 - .380
91.83 - .370
99.70 - .360
108.49 - .350 very common on 9,3mms
118.35 - .340 somewhat common on 9,3mms
129.43 - .330
141.95 - .320
156.14 - .310
172.28 - .300 very common on 8mms
These are most frequently seen with a / instead of the decimal point, as in 172/28 or 84/77

Thank you for that response. can you tell me what size shell caseing it would take the chamber looks like it will take a large bottle neck shaped caseing maybe with the 22 bullet in the end. It is extensivly engraved has has an engraved scope mount holds 4 shells under the butt has 4 holes for ammo with an engraved cover.it looks more like a fancy hunting gun it has a stag engraved on it also sry i left out this info.  

Answer
I have been researching this cartridge a lot and the only "POSSIBLE" answer I can come up with is the following.

I would take the weapon to a gunsmith and have them "Mike" the chamber to be sure.

6.5x52mm Carcano (Primary) /8mm Mauser/ 7.35x51 Carcano

A 6.5x52mm Carcano, a rimless, necked smokeless powder military cartridge. Adopted in 1891 for the Mannlicher-Carcano rifle. The Carcano 6.5 is very similar to the 6.5 Mannlicher-Schoenauer in dimension and performance.  

Specification   6.5x52mm Carcano
Bullet Diameter   .265
Neck Diameter   .295
Shoulder Diameter .430
Base Diameter   .445
Rim Thickness   .045
Case Length 2.05
Cartridge Length 3.02
Velocity 2296
Energy   1902
New Production Ammo: Ammunition is available from Norma.
Surplus Ammo - Difficult to find.
Reloading Dies are made by Lee Precision .
Reloading Components can be found at Graf & Sons.