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throat length

31 Mar 2011
@ 01:28 pm (GMT)

walter hicks jr

In the .270 win knowledsge base it states powder choice is very much dependent on throat length. Short throated .270 caliber rifles simply cannot obtain optimum results with slower burning 4831 type powders. What is the length of a short medium or long throat? Is there a set rule or does it vary by caliber and cartridge. I know it varies with bullet ogive. I have 3 30 06 each a different manufacture and each has a different throat length so if there is a standard it is not followed.


01 Apr 2011
@ 10:01 am (GMT)

Nathan Foster

Re: throat length
Hi walt, SAAMI members have a set reference for COAL's and ammunition manufacturers stick to these guide lines.

As you have noted, rifle manufacturers sometimes do not follow strict guidelines. There are perhaps a few reasons for this. With cartridges like the .30-06, magazine box length of reworked military actions dictated the COAL's of both ammunition and chamber design. As for the .270, I believe that to some extent (I may be wrong), what we sometimes see, is a chamber cut with a reamer that has been re-ground midway through its life. A fresh reamer will cut the longest throat, a reharpened reamer will cut a shorter throat.

The worst .270 rifle I have seen was produced by Browning (Japan). The throat was so short that factory ammunition was within 5 thou of the lands. Max pressures were reached with factory ammo. Max COAL for handloads was 79mm (3.110"). At the other extreme, were the Sako AV and Finnbear which had max COAL's of 87mm (3.425"), a huge difference.

The Japanese do seem to re-grind their reamers. I have seen variations from the 79mm mentioned to (looking at my client notes here) 80.5mm, 83mm, etc etc.

With U.S made rifles, the variations seem to be far smaller with .270 max COAL's hovering around the 85mm (3.346") mark, giving best results with 4350. I believe Sako were the only rifle maker to really let the .270 shine. Back when everybody was producing 22" barreled rifles, Sako ( thru 1970's-1980's) made 24" stout barreled rifles allowing for Max COAL's of 87mm and the reloader could utilize 4831 and the long barrel, to drive 150 grain bullets at 2950-3000fps. For a non magnum cartridge, this was pretty potent, especially when using the 150 grain Hotcor or Partition. Compare this to the other extreme, the Browning, achieving max velocities of 2800fps with hand loaded 150 grain bullets. (I am using the averages of the Interlock/GameKing/Hotcor as my reference for the COAL's quoted in this post).

For modern rifle manufacture, chamber dimensions are a bit of a catch 22. A loose chamber and long throat help ensure that the consumer won't blow himself up and cause a law suit. On the other hand, a tight chamber and short throat ensure good accuracy and high velocity with factory ammunition. I am sure rifle manufacturers would have spent a lot of time contemplating and experimenting with these factors during the introduction and early production of the WSM's.

There is also the factor of tradition. The standard .308 chamber design has huge freebore, bullet jump is a good 4mm (.157") and its impossible to get close to the lands if seating ammo to work through typical box magazine lengths of 72mm (2.834"). This design is possibly a hangover from military chamber designs and works fine with the .308 design.

When building a custom rifle, the end user has the opportunity to tailor throat length to their liking, using a seperate throat reamer. If you approach a reamer maker such as PTG, they have standard chamber designs. Beyond this, the end user can create further mods.
01 Apr 2011
@ 03:16 pm (GMT)

walter hicks jr

Re: throat length
Hi nathan and thank you for your reply. You answered a couple of my questions. As to the reaason for my post, of my 06s the worst performer is my 700bdl which has the longest throat with at least .120 jump to the lands at recomended coal. Speer has the shortest jump and berger the longest jump. My t/c encore has the shortest throat. I have also a t/c encore in .270 win and bullet jump is as follows sierra 135 hpbt match 3.382 to the lands with sierra wanting 3.340 coal. Sierra 150 sbt at 3.347 to the lands with sierra wanting 3.315 coal. Speer 130 hot core at 3.200 to the lands with speer wanting 3.240. This is way short. Finally, hornady 130 sst 3.331 to the lands with hornady wanting 3.180 coal. This barrel is also hard to find a load for. With the throat length you have spoken of I think mine is a short throat and I should use faster powders rl 17 and rl 19. My best so far has been 1.5 " at 100 yds with rl 22 and sierra bullets.


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