APS Hybrid throat test results.....

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Fiftydriver, Sep 9, 2008.

  1. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

    Jun 12, 2004
    To all,

    I have been working on a new throat design attempting to increase the performance level of a given chambering over what is conventionally available. Shawn Carlock has also been working hard on a similiar idea and both of us have been getting very good results on some very unique throat designs. Our designs are a bit different but the goal is the same, get more performance from a certain chambering.

    I have tested some different chambering with my "Hybrid Throat"(HT) but they were all seasoned barrels. In those tests it did appear that the velocity gains were in the 125 to 150 fps range with same bullet weights and same barrel lengths.

    This most recent test was with my 338 Allen Xpress which I posted pics of earlier. Here are some of the early test results. I will not include specific load data for one reason, it would not be safe with other 338 AX rifles with conventional throats.

    I started this test with a freshly machined barrel, broke it in and then did the velocity and accuracy testing. Remember that the barrel length on this rifle is only 26" in length. The throat length in this barrel is extremely short with an overall length of only 3.600" with the 300 gr SMK. This means it will easily fit in ANY magnum length receiver easily. I could lengthen the throat length a bit as even the very short Badger Ord DM system will allow 3.650" OAL, still I wanted to take the legs out of this barrel as much as possible to see the difference the new HT design would help us.

    I started with Retumbo and the 300 gr SMK which I have found to be the best all around powder in my 338 Allen Xpress.

    Conventional throat, 3.600" OAL
    Top velocity was 2760 fps
    Slight ejector ring on case head, easy extraction, very tight primer pocket

    Hybrid Throat, 3.600" OAL
    Top velocity was 2816 fps
    Absolutely no pressure signs at all but I ran out of case capacity

    I had to use 4 grains more powder with the HT and it did produce 56 fps more velocity. I was frankly suprised at the lack of pressure sign and obviously Retumbo was to slow in burn rate for this throat design, bullet and chambering.

    To test things a bit more I decided to try a powder that was a bit faster burning but would allow much more powder to be used. I used Ramshot Magnum which is a ball powder. For the same grains of powder compared to Retumbo, Ramshot Magnum produced slightly more velocity with a 2853 fps average and again no pressure signs at all. The good news is that there was still room in the case for more powder so up we went.

    When I topped out with a slight but noticable ejector ring on the case head, easy extraction, very tight primer pockets, the average speed was 2909 fps!!!!

    Again, remember this is in a 26" barrel length and an extremely short 3.600" OAL seating depth which with the 300 gr SMK, takes up ALOT of case capacity. In fact, seated this deeply, even on the improved Lapua case, the case capacity is slightly less then that of the standard 338 RUM with the same bullet seated to 3.900".

    Give some persective on what the throat does for velocity.

    perhaps the most interesting part, the accuracy test. I shot five, three shot groups at 100 yards with the conventional throat with the load that produced 2760 fps. The average for those 5 three shot groups was 0.488" ctc.

    After testing the HT chamber I loaded up a batch of ammo at the 2853 fps level and shot five three shot groups again to test accuracy levels. Again at 100 yards. The average was 0.320" ctc. Dramatically tighter groups.

    The rifle shot very well with the conventional throat but in most groups there would be two touching and then one shot that would open the group up slightly, not much, still under 1/2 moa average easily but there was always one shot that would pull off the other two.

    The groups with the HT chamber were dramatically more consistant. In all groups, all shots were touching, no slight fliers at all.

    Thinking about why this happened, I simply believe that the HT design imposes less stress on the bullets jacket and also imposes less stress to the core/jacket bond. Even though in both loads, the bullet is seated right to the origins of the lands, there is hardly any engraving force needed to start the bullet into the bore with the HT.

    In fact when setting up the seating depth, I really had to watch for marks on the bullet because I could not "feel" the lands contact the bullets like you can with the conventional throat.

    Its a bit different to work with, need a bit faster burning powder and care taken to seat the bullets to the correct depth but from these results, I am very happy with this design.

    Next test, long range accuracy testing and to see how long the throat design holds as its shot more. If the throat live is not there, there will be no advantage to the system. Time will tell but for now, I am very happy. Now I just have to get alot of bullets down range to see how the throat holds up.

    Not offering any details on the throat design yet simply because the design is not set yet, may change it still.
  2. 300ECHO

    300ECHO Active Member

    Aug 3, 2008
    VERY COOL!!! let's hope it holds up to the test, I take it that you did it with low angle lands or throat has 3 then has normal 6 lands farther up?IMPUT:) ?

  3. remingtonman_25_06

    remingtonman_25_06 Well-Known Member

    Jun 4, 2003
    Thats great news Kirby!!

    Is this throat design going to be available on any caliber the customer wants to be built, or just for your AM cartridges??

    Hope to see great results at LR with accuracy, and overall with the life of the throat as my next build I would like to be by you with this throat design if possible. Although I dont think my build will be a AM chambering, thus my question about the other options of calibers.
  4. ss7mm

    ss7mm Writers Guild

    Jun 11, 2005
    Ah yes, always pushing the envelope.;)

    Hey, it only seems fair to me that you do all of the work and we get the benefits of your successes.

    Wait a minute......we end up paying for your playing in the end don't we???:confused:

    Still, that only seems fair to me. If it wasn't for guys like you we'd still think 500 yards was a looong shot.;);)

    Keep us posted as it sounds promising if the barrel/throat life is there.
  5. Varminator 911

    Varminator 911 Well-Known Member

    Dec 7, 2007
    Guess I'm thinking you must be tapering the throat somewhere between
    conventional and a freebore. Maybe you've already said that. Anyway
    this sounds interesting.
  6. 300ECHO

    300ECHO Active Member

    Aug 3, 2008
    gain twist(cough)???sorry I'm nosey! extra 100fps on next build would be awesome, great work however you got there Kirby!!!
  7. Franklin

    Franklin Well-Known Member

    Feb 23, 2008
    Hmm...sounds like you're cutting the throat to introduce the bullet to the lands very gently. Hope it works out for you, what ever your doing.:cool:
  8. lazylabs

    lazylabs Well-Known Member

    Oct 26, 2006
    So the obvious question.... erosion results? I know it's probably too early to tell but is the trade-off a faster moving seating depth?
  9. phorwath

    phorwath Well-Known Member

    Apr 4, 2005
    Sounds interesting.

    If all else fails, there's always the former option that was promoted by wildcat. More powder and then call the contraption Varminter. Ha! Just kidding. Hope you develop something here that provides some real benefit.