Throat length, does it really matter that much?

[Fiftydriver]

Well, I have been wanting to do this experiment for a long time. Over the years, I have seen to many short throated rifles put up velocity numbers that would just make many out there question the results. So, I have always wanted to do a head to head comparision to see what throat length really did.

Now, probably the very best way to do this would be to build a rifle chambered with a short throat, do load development with a pressure guage to get pressure data and then record velocity and then cut the throat longer and repeat and see what the results were when the longer throated chamber was loaded to the same exact pressure.

Well, I simply do not have time to do that right now but I had a unique opportunity fall into my hand the past couple weeks. Last week I finished up one of my Raptor LRSS rifles in 338 Allen Xpress for a customer. 30" Lilja, 1-10 twist. Breech bore diameter was 0.3304", muzzle diameter 0.3302".

This rifle was a single shot and the throat length was such that a 300 gr berger OTM was seated to 3.950" to touch the lands. I used RL33 for this test and started at 100.0 gr and worked up 1/2 grain at a time until the case heads went from absolutely no ejector ring of any kind to just the SLIGHTEST hint of a ring with a 1/2 grain increase. To confirm this, once I got to this point, to confirm I loaded up three rounds at that load and the one load at less by 1/2 grain to make sure that this was really the point where the ejector ring just started to show and not just a single case that was a bit softer then the rest of the cases.

With this rifle, that point was going from 109.5 grains to 110.0 grains which produced an average velocity of 3041 fps.

Today I was able to do load development on another Raptor LRSS with Lilja 1-10 twist, 30" barrel. Breech diameter was the exact same 0.3304" and muzzle diameter was also the exact same 0.3302".

The only difference was that that this rifle was a repeater set up to use a Badger Ord detachable magazine system with the 300 gr berger OTM seated to 3.740" OAL.

In both tests, the same lot of powder was used. Same lot of brass, same lot of bullets.

Now, I was expecting that RL33 may be to slow burning for this short OAL with the big 300 gr OTM taking up to much case volume to reach top working pressures. Still for the sake of getting the data, I wanted to give it a try even though I thought that Retumbo would be a better match for this rifle.

Because of the reduced load capacity, I started at 95.0 grains of RL33 which proved to be VERY mild. Again, the test was to increase load 1/2 grains at a time until the faintest ejector ring just became present, same as the first riflle. To be sure not to influence the results while I was performing the test, I did not shoot the rounds over the chronograph. Instead I simply shot the rounds until I got to the point where the ejector ring JUST started to appear.

The first test was done at 70 degree temp and today was within a couple degrees of that test temperature.

This short throated rifle topped out at 107.0 grains of RL33. That was not a surprise, the shorter OAL has to reduce case capacity. The suprising thing came when I chronographed that 107.0 gr load. It average velocity was 3037 fps. 4 fps less then the long throated barrel with what appeared to be nearly identical pressures.

So..............

We have always been told that you could take a round and by increasing the length of the throat, you could increase velocity and while this may be true to some degree but in this case, it clearly shows that throat length is not all its cracked up to be. Just because you add 0.200" of length to your OAL in ammo, do not expect that you will automatically increase in velocity.

In this test, when both 3.740" and 3.950" ammo were loaded to apparently the same chamber pressure, velocity was nearly identical.

One interesting note however, the 3.740" length had around a 104% load density. The 3.950" length was around 98% load density. Yet, chamber pressure was as close as I could get it with the means I was testing with.

Now, I am sure some of you will simply dismiss this by saying that obviously the shorter round was simply loaded to higher pressures...... All I can say that is Each of these tests produced the exact same FAINT shadow ejector ring on the case head with only a 1/2 increase in charge weight. The load before with each load, there was NO shadow ring of any kind.

I have done similar tests with my 7mm Allen Magnum when I was doing the very first load development for that wildcat and using the old Wildcat Bullet 200 gr ULD RBBT seated at 3.600" and at 3.900" with very similar results.

I have just seen to many short throated rifles come very close to matching long throated barrels to not think that throat length has less effect on velocity then we may think. My belief is that as long as a given case has the appropriate case capacity to use a specific burn rate of powder to allow 95 to 105% load density with that powder, it will produce very similar top end velocities no matter what the throat length that is used.....

Now, with all this said, I have seen several instances where seating a bullet dramatically longer resulted in a case capacity that allows a round to step to a different burn rate class of powder. In this case, yes, velocity output can be quite different. But using the same powder with same bullets in same length barrels and loaded to same chamber pressures, the difference may not be as large and many of us have been lead to believe.

One note to mention, the throat design in both rifles was identical in that both were 0.3385" diameter and both have a 1-30-00 lead angle.

Take if for what its worth but in this test, throat length did not appear to offer much of anything as far as velocity output potential.

 

[Mikecr]

Well, when you normalized pressure, you essentially made them the same -except using a different amount of powder to compensate for different throats.
Now you could do the opposite and use the same amount of powder, and measure veolicites -with a short throat -vs- long throat,, and with this the results would imply pretty much what anyone expects of the condition.

 

[MudRunner2005]

This is great news Kirby! That means my gunsmith had it right all-along. He thinks that special throating for certain bullets is overrated, and only does it at request for custom BR rifles and such, unless you specifically request it on a hunting rifle. Even then, he'll try to dissuade you from doing it.

Thanks for this test and posting your results.

 

[Broz]

We just did this Kirby with the same barrel, and results are very close to yours. .330" shorter throat to touch the lands. On par for the same velocity with a little higher percent of case fill on the shorter throat. Same powder lot. Same bullet lot, same primer lot and same barrel. Our test was in no way scientific and could easily be criticized by scientists, but for what its worth it was similar to your results.

Thanks for sharing your results.

Jeff

 

[Broz]

I will add my personal thoughts here for whatever it's worth. I feel throating for the bullet does have some merit. But we should also not over look the results that seating depth has for the relation to case fill in this matter. I want to try this same combination back out with the bullet another .100". The reason is as I test this combination , with this exact powder and bullet, I do feel there is a "perfect sweet spot" for this chambering, bullet and powder charge. With enough testing I feel this will fine tune it to a close to perfect combo.


Jeff

 

[Edd]

I think that test showed exactly what you would expect additional throat length to show, it made the chamber larger and allowed the use of more powder.

I think getting only an additional 4 fps (.13% increase) from an additional 3 gr (2.8% increase) of powder is a separate discussion.

 

[dkhunt14]

We took a WSM that pressured up real quick and we couldn't get the velocity needed for 1000 yard and it was real hard on brass. The Berger Pressure ring on the 210 VLD was setting dead even with the junction of the neck and shoulder. We got around a hundred increase in velocity with the bullets 10 in with moving the throat out so the pressure ring was in the middle of the neck. It took almost 2 more grains of powder but we went from 2800 with pressure to almost 2900 without pressure. The components were the same. Matt

 

[KYpatriot]

Is there enough barrel to get complete burn of the extra 3 gr of powder? I assume 30in would be enough, but if not perhaps the extra 3 gr was wasted and that's why you only saw an additional 4 fps.

It would great to do your test with the pressure trace system. I have thought about buying one myself just out of curiosity. I wonder sometimes if we might be surprised by what we think we know about chamber pressure by reading brass. So far spending $600 of the gun budget to buy a strain gauge out of mere curiosity seems a little much for me since I am not pushing the boundaries of the sport or creating new high performance rounds like some of y'all are. It would be really cool though to correlate reliable chamber pressure data from a strain gauge with our observations of brass condition ( and velocity data) to see where the truth lies in reading brass. Any ever used that pressure trace system?

 

[Fiftydriver]

Well, when you normalized pressure, you essentially made them the same -except using a different amount of powder to compensate for different throats.
Now you could do the opposite and use the same amount of powder, and measure veolicites -with a short throat -vs- long throat,, and with this the results would imply pretty much what anyone expects of the condition.

Not sure I understand what your saying here. Both test rifles were loaded to the same top working pressures for the chambering being used so yes your correct that the pressures were the same, that was the point of the test.

We have been told all along that you can seat your bullets out farther, increase your case capacity and use the same pressures but because of the increased fuel in the case, you will get more velocity, that does not always play out to be a fact.

If I used the same amount of powder in the short throat as I did in the long throat, the pressures would skyrocket in the short throated chamber. If you read the report, 107.0 gr with the short 3.740" OAL produced around a 104% load density. The 110.0 gr load in the 3.950" OAL round produced around 98% load density at the same pressure as the short round.

The point of this test is that both pressures have to be the same. I think your missing that point.

Hell, I could load up a faster powder in the short OAL ammo and run it to redline for the Lapua case and kick the pants off the longer OAL ammo using RL33. what would be the point of that test though.

Personally, I believe that's how most people get more performance out of a round that they use with a long throat. They simply push the throttle down a bit harder to get the performance increase they THINK they should get.

Again, not sure you understand the point of this test, at least your reply does not seem like you did.

 

[Fiftydriver]

We just did this Kirby with the same barrel, and results are very close to yours. .330" shorter throat to touch the lands. On par for the same velocity with a little higher percent of case fill on the shorter throat. Same powder lot. Same bullet lot, same primer lot and same barrel. Our test was in no way scientific and could easily be criticized by scientists, but for what its worth it was similar to your results.

Thanks for sharing your results.

Jeff

Ya, my test was certainly not scientifically pure either but both of our results show similar results. I would have to agree with you also in that each round may have a Sweet spot for throat length to optimize case capacity for a given powder charge. For example, you may be able to take a given case and put X amount of powder in that case with a 3.700" OAL, 3.800" OAL and 3.900" OAL.

With the short length you may have 105% load density and a bit more powder crunching then we like to see, at 3.900" we may have around 95% load density.

At 3.800" we may have a 100% load density which would be ideal for most powders.

Agree with your comments for sure.

 

[Fiftydriver]

I think that test showed exactly what you would expect additional throat length to show, it made the chamber larger and allowed the use of more powder.

I think getting only an additional 4 fps (.13% increase) from an additional 3 gr (2.8% increase) of powder is a separate discussion.

It made the chamber volume larger but had to use more powder to get to the same apparent top working chamber pressure with no increase in velocity however.

Remember I did the test on the short OAL rifle blind. I did not shoot over a chrono until I reached the point where I first saw the very faint hint if an ejector ring just as I got with the long throated rifle so there was no possibility to push things a bit harder or hold back a bit to bias the test.

In this test, with the short OAL ammo, it took 107.0 gr powder to reach a given pressure level and 3037 fps. In the longer ammo, it took 110.0 gr of powder to reach the same apparent pressure and for the most part identical velocity. The added amount of powder did not result in any practical increase in velocity.

 

[Fiftydriver]

We took a WSM that pressured up real quick and we couldn't get the velocity needed for 1000 yard and it was real hard on brass. The Berger Pressure ring on the 210 VLD was setting dead even with the junction of the neck and shoulder. We got around a hundred increase in velocity with the bullets 10 in with moving the throat out so the pressure ring was in the middle of the neck. It took almost 2 more grains of powder but we went from 2800 with pressure to almost 2900 without pressure. The components were the same. Matt

I would say you got the increased velocity simply because you removed the pinch point between the neck of the case and the bullet and had very little to do with the longer OAL and the additional two grains of powder used.

In fact, I would bet that there is a minute donut at the base of the inside of your case necks that was causing more of a problem then the pressure ring on the berger bullets which is very minimal at best.

This is one reason I do not design my reamers to have extremely tight necks. Can cause these little issues that you saw and the more extreme you get in performance the more you will see the effects of these things in velocity and pressure. In the 300 WSM which is about as mild and user friendly as it gets, you do not see much of a problem. In something like a 7mm Allen Magnum or 338 Allen Magnum, it can be dramatic and cause severe pressure swings.

Again, I would bet that if you inside reamed your case necks or had a neck that was 1/2 to 3/4 thou larger in diameter, you would not see this problem with pressuring up. I believe you simply removed the pinch point and that resulted in freeing up some extra performance, very little to do with the actual longer OAL and two additional grains of powder. In my opinion anyway.

 

[Fiftydriver]

Is there enough barrel to get complete burn of the extra 3 gr of powder? I assume 30in would be enough, but if not perhaps the extra 3 gr was wasted and that's why you only saw an additional 4 fps.

It would great to do your test with the pressure trace system. I have thought about buying one myself just out of curiosity. I wonder sometimes if we might be surprised by what we think we know about chamber pressure by reading brass. So far spending $600 of the gun budget to buy a strain gauge out of mere curiosity seems a little much for me since I am not pushing the boundaries of the sport or creating new high performance rounds like some of y'all are. It would be really cool though to correlate reliable chamber pressure data from a strain gauge with our observations of brass condition ( and velocity data) to see where the truth lies in reading brass. Any ever used that pressure trace system?

You can burn a lot more then 110.0 gr of powder in a 30" 338 cal barrel!!!

I understand what your getting at and I have tested this a lot with some of my Xtreme XP-100 builds using 15 to 17" barrel length and in those cases, yes, you get to a point where you simply do not get any more velocity for more powder in the case.

In both of these tests however, ever increase in powder charge produces predictable, steady and even velocity increases. IF what you describe was happening, the velocity would have topped out at 107.0 gr with both of the rifles and in both tests, each had velocity increase consistently with each 1/2 grain of powder charge.

I have done pressure tests on the 338 ALLEN XPRESS with the 300 gr berger in my 30" Raptor LRSS rifles to test the strength of my Raptor receiver and can tell you that a 30" barrel length will burn a lot more powder then most would believe!!! And the 338 Lapua case will take a lot more pressure then many will believe. They do not seem to be as strong as they used to be 10 years ago or so but still very strong cases.

It would be interesting to have a strain guage but from what I have read, they are not overly accurate. Pressure barrels are about the best way to measure pressure accurately from what I have read and been told.

 

[Fiftydriver]

Just as a side note, I took that rifle out this morning before the rain hit and it took me a grand total of 13 shots to get the rifle dialed in and drop chart proven. Let me explain, bore sighted the rifle at 1040 yards. Took three shots to get point of impact to match point of aim at this range. At 1040 yards, I hit two different 1/3 moa sized target rocks with two shots.

Customer wanted to zero the rifle at 200 yards and use dial up method for shooting. So, set the vertical adjustment turret to zero, ran the numbers or the conditions and vital performance numbers for that rifle/load. IT told me that with a 200 yard zero, the scope would need 19.9 moa to be dead on at 1040 yards.

So, I took 20 moa out of the vertical adjustment and reset the zero on the turret.

Ranged a 1/4 moa sized rock at 673 yards. Ran the numbers. Dial up was 9.9 moa, ran the turret up 10 moa. First shot clipped the top edge of the rock, second shot turned it to gravel. The two shots were under 3" ctc.

Returned the turret to zero.

Ranged a rock at 887 yards. Dial up was 15.5 moa. Cranked that in. First shot clipped the right edge of the 1/3 moa sized target rock. Second shot clipped the top edge.

Rezeroed the turret.

Ranged another target rock, this one a bit bigger, roughly 1 moa long by 1/2 moa tall, at 911 yards. Dial up was 16.2 moa, cranked in 16.25 moa. First shot landed about 1/2 moa high of center of the rock. Recoil felt funny to me, did not come smooth straight back like normal. Took a confirming shot and broke the target rock.

Rezeroed the turret.

Went back to the 1040 yard target distance. Dialed up the 20 moa. First shot landed on the bottom center edge of the 1/2 moa sized target rock. Second shot clipped the top left edge of the same target.

Drop chart was dead on from 671 to 1040 yards with less then 15 rounds down the barrel. I will test it at 200, 400 and 500 yards and also at 1200 and 1500 if I have time but its amazing how fast and easy it is to set up the drops with the equipment we have these days in computers and known ballistic data for our bullets. We are pretty lucky to be playing in our sport today!!!