Few days ago, at a local gun show, I saw a really big (and I mean HUGE!!!) old Mauser bolt action rifle. It looked like and oversize Gewehr 98.  I asked around, but I couldn´t find anyone who could tell me anything very clear about it. For the size of the bolt, I guess it fired a cartridge larger than a .50 Browning. Do you know anything about this?
I´m really curious about it...

It is most likely a WWI anti-tank rifle.  I read a magazine article several years ago which described their use and ballistics.  Let me do some research and I will post what I find.

Did it look like this?  www.midwestordnance.com/mauser1918.htm

With the first appearance of tanks and other armoured fighting vehicles arose the need for infantry anti-tank weapons. Late in World War I, Germans introduced heavy anti-tank rifle - 13.35mm Mauser Tank Gewehr Model 1918 based on 7.92mm Mauser Model 98 rifle. The new rifle was able to penetrate 26mm of armour at 100m and approximately 10mm at 200m. Mauser produced some 15800 but they didn't prove to be successful being heavy (approx. 18kg) and having very strong recoil. In addition to model 1918 rifle, Germans tested 13.35mm MG18 heavy machine gun. It was not until 1930s, when development of infantry anti-tank weaponry started again.

The two main anti-tank rifles used by the German Army were 7.92mm Panzerbüchse (PzB) model 38 by Rheinmetall-Borsig and model 39 by Gustloff Werke. Only 1600 PzB 38 were produced from 1939 to 1940, as the weapon was too complex and expensive. Lighter PzB 39 was mass produced (some 39232) and was widely used on all fronts. PzB 38 weighted 16.2kg, while PzB 39 only 12.6kg. Both anti-tank rifles had similar characteristics - muzzle velocity of 1210mps (PzB 38) vs. 1265mps (PzB 39) and length of 1.615m (PzB 38) vs. 1.620m (PzB 39). Both rifles were single shot weapons and fired the same ammunition (steel core and from 1940, tungsten core). The original bullets had hardened steel core and tiny capsule of tear gas. The idea behind the capsule was that once the bullet entered the armored vehicle it would disperse and force the crew to leave the vehicle. It didn't work as only the core penetrated and capsule was left outside. PzB 38 and 39 were able to penetrate 25mm of armour at 30 degrees at 300m. It was planned that each infantry division will have 81 rifles but it varied from unit to unit. PzB 38 remained in service in the early war years, while PzB 39 remained in limited use until 1943/44. Majority of PzB 39 anti-tank rifles was converted to grenade launchers - Granatbüchse Model 39 (GrB 39).

The rest of the article is here.  www.achtungpanzer.com/pzb.htm

And from www.geocities.com/Augusta/8172/panzerfaust6.htm


Panzerfaust
WW II German Infantry Anti-Tank Weapons
Page 6: Tank Rifles

Panzerbüchse

The idea of the Wehrmacht behind the Panzerbüchse ("tank rifle") was to give the infantry integral limited anti-tank capability against light tanks without having to wait for the tank hunter units to relieve them. However, the concept of the Tank Rifle was quite out of date for the tanks of WW II; the unsatisfactory performance of the tank rifles due to the spectacular successes hitherto went hardly noticed until the battle for Moscow in 1941 where the the weapon's proved themselves practically useless against the soviet tanks. Reportedly, many Panzerbüchsen were thrown away by the frustrated soldiers. Even earlier the weapons themselves were for a variety of resons not very popular among the troops, not the least of the reasons was that collar-bone fractures among the gunners were all too common.

Tank Rifle Ammunition

The success or failure of a tank rifle is largely depending on the ammunition it uses. Large caliber rifles that used high energy hardened projectiles - anti-tank rifles - had already been developed and used during WW I where tanks appeared on the battlefield for the first time in history. The first german tank rifle used then was the so-called T-Gewehr ("T-Rifle", T for tank).

During the 1930ies the company Rheinmetall used this weapon's ammuntion as a basis for the development of new tank rifle ammunition. Set into the old T-Gewehr casing was a 7.9mm projectile instead of the original 13mm. The complete cartridge weighed 84 g, the bullet weighed 14.6 g (as compared to the original bullet's 52g!) and had a steel core. The amount of gunpowder was increased to 14.9g; all this changed the V0 from the original 780m/s to now over 1,150m/s.
This new ammunition was called Patrone 318, the full official designation was Patrone 318 SmK-Rs- L'spur or Patrone 318 SmKH -Rs-L'spur, whereas "Patrone" means "cartridge", 318 was an inversion of 813 that stood for an 8mm bullet in a 13mm casing; SmK meant "Spitzgeschoss mit Kern" ("pointed bullet with core"), SmKH = "Spitzgeschoss mit Kern (Hart)" ("pointed bullet with core (hard)") meant the same projectile that featured a hardened steel core (some sources report a tungsten-carbide core but this seems wrong), Rs stood for "Reizstoff" ("irritant agent") because the projectile also contained a small amount of tear gas, L'spur for "Leuchtspur" ("bright trace" = "tracer") indicating the bullet had a small tracer in its rear. At a typical Vo of slightly over 1,200m/s the projectile penetrated 30mm of steel at a range of 100m and still up to 25mm at 300m (both at 0° slope) and was accepted as the standard anti-tank rifle ammunition to be used by all weapons of that type. The irritant agent in the projectile was a ridiculous idea that envisioned to make the crew of the hit target leave their vehicle or otherwise impair them to a degree of battleunworthiness; however, the tear gas pellet in the projectile was so little that the irritant agent was never noticed by enemy crews; in fact, it wasn't discovered by the allies until after captured ammunition was examined.
Production of the Patrone 318 ran until August 1942; overall, 9.417 Mio. cartridges were produced. Compared to the tank kills the same explanation as that given in connection with the Panzerfaust can be applied: many of these were not shot at tanks but at a variety of other targets.

The cartridge used for the PzB 35(p) was the Panzerbüchsenpatrone P 35 7,92 x 107. It weighed 62.6g which includes the 10.4g of powder; the projectile (which lacked a steel core) weighed 14.5g (other sources: 12.8g). total cartridge length 132mm, case length w/o projectile 107mm. When fired from the PzB 35(p) the projectile had a Vo of 1,275 m/s which equates to an Eo of 11,786 Joule.

The russian tank rifles PTRD-1941 and PTRS-1941, which were also used by the germans as the PzB 783(r) and PzB 784(r) respectively, used the russian M 41 cartridge that was later improved to the heavy machine gun and tank rifle cartridge M 41/44 14.5 x 114. The latter cartridge was also used by the KPV 14.5mm heavy machine gun (which made it a reasonably effective AT weapon against lightly armored vehicles) towards the end of the war. The cartridge is still in production today as a heavy machine gun ammunition. It has a caliber of 14.5mm; different projectiles were available ranging from the armor-piercing steel core to armor piercing incediary and explosive projectiles. The armor-piercing projectile weighed 63.4g, the whole cartridge weighs 198.5g incl. 28.8g of powder charge. Total length of the cartridge is 155.5mm; at a typical Vo of 1000m/s this ammunition has an initial energy of 31,700 Joule. It penetrated 40mm of armor at 100m; 35mm at 300m; 30mm at 500m; an AT rifle with this ammunition could still penetrate a light AFV's armor of 15mm at ranges of 1.5km, if the gunner was able to hit a lightly armored target at that distance....

I really thank you for the information!!
Very interesting,indeed. It was truly an awesome weapon.
Thanx!

I've seen a 98 converted to 12 gauge with a 2 shot mag.

Quoted: I've seen a 98 converted to 12 gauge with a 2 shot mag.

I've got one of those, but I would not shoot the specimen in my possession for safety reasons.  It was left to me by my grandfather, and it is the only "Wall Hanger" I own.  

Could you post some pics of the bolt face and action?

I'd be happy to if I had a digital camera!  I'll ask a few people I know if I can borrow theirs, but it may take some time.

The bolt face is held in place by a modified extraction claw, and actually detaches from the bolt when the bolt is removed from the receiver.  

The receiver appears to be cobbled back together from a demilled, possibly WWI era, receiver.  The magazine box, follower, and feed rails have been reworked for 2.75" 12 ga shells.  The only marking on the receiver ring is "Germany" on the left side, just above and parallel to the stock.

The barrel has 3 proof marks and the word "Nitro".

The stock appears to be walnut and has a circular medallion on each side with the word "Geha" in cursive script in the middle, but the medallion on the right side is missing from my specimen.

Many of the anti-tank rifles are curios, but ammo is hard to find naturally.
I hope Russia will start importing some of the old German ones soon.