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7mm REM longthroating

28 May 2012
@ 10:59 pm (GMT)


Like so many things once one question is answered it leads to another more interesting one.

For a brand new 7mm RM Remington 700 SPS I have 100 Winchester cases (Nosler on order), 50 162 gr SSTs, CCI 250 primers and RL25 powder. I measured the length to ogive and found that I have a lot of projectile protruding into the case. I do remember reading somewhere in the KB that the only real difference between the 7mm Weatherby and the 7mm RM was the free bore in the WBY. I have read elsewhere that the free bore can induce slight inaccuracy due to free travel of the projectile.

Given I have well over 3/8" of projectile past the end of the neck I could have the 3/8"of free bore and still have the projectile fully engaged at any point.

That brings me to another point to consider. I might gain fps by free boring and reducing the amount of projectile seated into teh case, but I also loose a bit by shortening rifled length of the barrell.

With all this said, is it worth it to extend the free bore of the rifle to a length that still ensures the selected projectile is still engaged at all times.

Thanks in advance.



29 May 2012
@ 10:11 am (GMT)

Nathan Foster

Re: 7mm REM longthroating
Hi Danny, in my own tests, every 1mm (40 thou) cut into the 7mm RM equated to 50fps. If you wanted to play it safe, 30fps would be a good number. So lets say your 162gr SST MAX COAL is 84.8mm, by cutting 2.5mm forwards, you would gain 3x30= 90fps and up to 3x50= 150fps.

There are a couple of points to bare in mind though. The Weatherby case has a long neck to aid concentricity,lining the bullet up squarely to the bore and guiding it for quite a distance before the bullet makes its final leap from the case neck to the rifling. Without this long guide, for the sake of accuracy it is better to have a shorter bullet jump.

The next factor to bare in mind is that the 162gr SST is only one bullet design. If you start playing with the A-Max, you'll see that the max COAL is 1.5mm longer and that the seating depths versus powder capacity are about right.

Optimum throat length for the 7mm Rem Mag should place the A-Max at a max COAL of around 87.6 to 87.8mm (remove 1.5mm for your SST). This allows some room for throat wear/growth.

Occasionally, I will come across a short throated rifle (Remington brand included) where the max COAL ends up at 86.3mm. Thats quite a discrepency in machining tolerances when you look at it, but as far as velocities go the bore tolerances usually have the final say.

If you do find that yours is a short throated rifle, you can increase the throat length if you like. Lets say your max COAL with he SST is 84.8mm (U.S readers please divide by 25.4 for imperial) and optimum velocity/accuracy is 2960fps. By cutting to a max COAL of 86.3mm, the rifle will potentially produce 3050 to 3110fps. The figure of 3070fps sits about right in my mind. Not really a great difference. You also have to allow for a little bit of error for the smith to work with. Cutting leade angles is really difficult for smiths as by the time they bring the tool up to touch and feel the cut, the cut can already be well underway. So it pays to leave an error tolerance of -0 +.2mm. That helps keep the stress levels down.

Now, if you really want all out velocity, then the best bet is to go straight to something like the 7mm Practical. This reamer will cut the old chamber right out without having to move the chamber forwards by more than a few mm and potentially losing what little knox form the SPS already has. By this I am referring to the rapid tapering of the barrel contour, you don't want to lose any more meat at the chamber end. The 7mm Practical is able to run the 162gr SST up at 3200fps or higher. From the SPS platform, about 3225fps is as fast as the platform will handle without excessive recoil effecting long range accuracy.
30 May 2012
@ 12:27 am (GMT)


Re: 7mm REM longthroating
Pitty I already bought the dies for the 7mm RM. Maybe if I shoot the barrell out the 7mm Practical will come into play.

As for now, the head to ogive length is 69.6 to 69.5mm. The total length from tip to head seems to be 85.2mm. It leaves about 9.78mm of the bullet's (minus boat tail) contact surface in the neck and case. With a neck length of about 6.40 and allowing for .5mm jump, that leaves about 2.5mm free bore with the whole of the neck guiding the projectile.

What would happen if I allowed 3mm of the contact surface in the neck rather than 6mm thereby increasing free bore to 5.5mm?.

Is 3mm enough?
30 May 2012
@ 11:57 am (GMT)

Nathan Foster

Re: 7mm REM longthroating
Hi Danny, the .5mm jump is your free bore.

In other words, if you have a max COAL (touching the rifling) of 85.2mm and then seat to 84.7mm, the free bore is .5mm.

If you were to ream 2.5mm forwards to increase the throat length and then seat .5mm of the lands, the free bore would still be .5mm.

If you were to ream for Weatherby style freebore, your max COAL with the SST would be (just an example) 95mm. (open your verniers up to 95mm and sit your case and SST in the gap to see what I mean). You would then have to seat to 89mm for concentricity (bullet boat tail junction flush with case neck shoulder junction). The free bore/bullet jump would now be 6mm.

Hope that makes sense.

The 7mm Practical uses a 7mm RM neck die and a 7mm RM seating die, Hornady brand being the best for this purpose. If you were to start reaming, this is the way to go. But lets work with what you have. The SPS is a light weight platform and you will have your work cut out for you learning to master magnum recoil as it is- let alone getting the bedding sorted, the trigger sorted and the bore lapped and run in properly. A load giving 2900fps is ample for 1000 yard shooting, anything else is a blessing and no doubt your rifle will yield atleast 2960fps.
30 May 2012
@ 12:15 pm (GMT)

Nathan Foster

Re: 7mm REM longthroating
One more thing to clarify. From a machinists perspective (including reamer manufacturers), free bore starts at the case mouth and is measured forwards to the rifling.

From a shooters perspective, free bore refers to bullet jump, measured from the ogive of the projectile forwards.

Same term, two completely different uses of the term.

Going into a bit more depth (for those who want a headache):

Reamer manufacturers use the terms free bore and lead (or leade) interchangeably. In other words, free bore and lead both refer to the area from the case mouth to the beginning of the rifling taper.

Reamer manufacturers use the term throat to describe the ogive shaped part of the reamer. This describes the length of the taper before it comes into contact with the projectile. The throat of a chamber has two important measurements (to the reamer maker), length and angle.

Try not to dwell on this as it can be confusing for industry outsiders. Reamer makers such as PTG are very smart in that if you as a consumer say that you want a long throated cartridge, they translate this into meaning a long free bore cartridge and will work towards this goal accordingly.

A throating reamer cuts both lead and throat angle.

But for all intents and purposes, please stick to the first two sentences of this post as these are the most relevant to your discussion.
30 May 2012
@ 10:30 pm (GMT)


Re: 7mm REM longthroating
Thanks for those top answers Nathan.

Is it worth:

giving the rifle a Weatherby frebore,
tapering the lead like on the 5.56mm military rifles.

And in general is it worth it all. Is the risk of reduced accuracy worth it.
31 May 2012
@ 01:12 pm (GMT)

Nathan Foster

Re: 7mm REM longthroating
Hi Danny, no its not really worthy the risk. The neck of the RM case is too short and is insufficient at guiding the projectile squarely to the bore if weatherby style free bore is used.

Thye throat angle you are talking about is for low drag bullets. The simple term is a VLD angle throat.

It is not really worth changing the throat angle on your rifle. The Rem factory throat angle works well with both hunting bullets and VLD type bullets. I do not know if Remington currently use a VLD angled throat but their current reamer design works very well.

Things you have to bare in mind, it costs the same to alter the throat as it does to ream a whole new chambering. So if you are going to have to pay eg $250 for 100fps, you might as well pay the same $250 and adopt a more potent cartridge.

But again, to be honest, the recoil of the 7mm Rem Mag is about as much as most folk can handle from the lightweight SPS rifle platform without getting into muzzle brakes. You already have some serious challenges ahead of you. Running in the SPS bore is in itself a challenge as the usual barrel break in methods don't apply to the somewhat rough SPS bore. Bedding and stock stabilizing are another challenge. Getting the trigger set up right is a factor, also having to decide whether you can get the X-Mark trigger down to 1.5lb safely and without creep versus whether a replacement is needed.

Following this will be your hand load variables and along with that, you'll really have to be on top of your shooting technique. The slightest flaw in technique will open groups up to 1.5" or so at 100 yards.
02 Jun 2012
@ 02:38 pm (GMT)


Re: 7mm REM longthroating

I note that when loading the magazine of the SPS 7mm RM one must stack the rounds so that the rim of each case is forward of the previous one so as not to catch on it and obstruct chambering a round. When a single round is placed in the magazine there is plenty of room to lengthen the throat to allow the projectile to protrude further from the case and not take up so much case space. Loading further rounds reduces this capacity to the point where at the third round I doubt the projectile can be loaded any futher out than the 84.50mm the throat is currently set to.

I'd like to hear comments on this.
06 Jun 2012
@ 12:04 pm (GMT)

Nathan Foster

Re: 7mm REM longthroating
Hi Danny, you are charging the magazine incorrectly. This is a common mistake for shooters new to the belted magnums. This is something you will have to pay close attention to for a little while. If you don't, you will get jams in the field- right when you don't need them.

Put the first round in the mag, thumb it down then thumb it back hard. You will note that this becomes a circular action of the thumb.

Do the same with the second round, thumb it down hard and back hard until the belt rides over the bottom cartridge. It will not jam behind the bottom cartridge as you thumbed the bottom one back to begin with. Now you can see the belts of the cases are stacked in line. Do the same with the third round. The action will now feed correctly. The action and magazine is not designed to be fed like an SMLE.

You will need to get into the habit of thumbing rounds down and back hard against the rear of the mag box otherwise you will have problems down the track. Same goes if say you have fired off three shots in the field and are in a rush to reload- recharging the mag is the one thing you can't rush, atleast until you have your methods down pat.

The internal mag length of your SPS mag box is 94mm. Cartridges can be seated as long as 93mm (in such chamberings as 7mmSTW) with smooth feeding. I run COAL's of around 92mm in my 7mm Practical which uses the same action as yours.
08 Jun 2012
@ 12:27 am (GMT)


Re: 7mm REM longthroating
Thanks for that Nathan. I explained how I wanted to load teh magazine and the machinist is clear on giving me .020"jump when the bearing surface is seated to teh junction of teh neck and shoulder. I got him to bed it also. Let's see how it goes.
08 Jun 2012
@ 09:51 am (GMT)

Nathan Foster

Re: 7mm REM longthroating
Please be careful with this. If you seat for 20 thou jump the way you describe, there is no room for throat wear. You have also left no room for experimentation should the SST need to be seated to 10 thou which is sometimes required in order to get the SST shooting straight. Also, you will not be able to seat the 162gr A-Max close to the lands if you set the rifle up specifically for the SST. It is more preferable to set the rifle up for optimum performance with the A-Max and let this decide where the SST will sit. If you don't, you may not be able to utilize this long range bullet should you wish to switch to it.

And regarding the bedding,make sure the smith beds the first 1" of the barrel. If he doesn't, the stock will flex up and pinch the front of the recoil lug with varying tensions. If the bedding is not done as described in our instructions, due to the SPS stock design, these rifles will sometimes shoot worse than factory pressure point bedding. Generally gunsmith bedding skills are going downhill world wide. It is therefore your responsibility to quiz your smith and ask him if he will bed the fiorst inch of the barrel and relieve the front bottom and sides of the recoil lug. There were some outstanding smiths in Oz back in the 60's through to 1980's. There is no excuse for a decline in workmanship when these guys paved the way so very carefully.



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