Reloading: IMR 4320

IMR 4320 - The Forgotten Powder
by: Germán A. Salazar

If you are relatively new to reloading, you may not have even heard of IMR 4320.  It is probably the least popular powder in the IMR line and yet, it has some great applications.  IMR 4320 falls right between 4895 and 4064 on the burn rate scale for most cartridges - in other words, right where Varget falls.  Yet, despite having a very useful burn rate, 4320 has languished for decades while newer powders of similar burn rate have gained in popularity.

I have an older lot of it (1998) which I use in the .308 with 185 grain bullets and it works very nicely for 500 and 600 yard matches with a mild charge producing velocitied in the low 2600 range.  I also use it with 168 gr. and 175 gr. bullets in the .30-06 for 200 and 300 yard loads.  However, I was interested in seeing what current production IMR 4320 would do in the .308 with the 155 gr. Palma bullets.  My interest was sparked by the obvious fact that Varget and H4895 have been very hard to get lately while 4320 sits on the shelves.  Lester Bruno told me recently that he sent a batch of IMR 4320 back to Hodgdon in the middle of the powder shortage because it doesn't sell.

A look at the Sierra manual shows that their maximum load with the 155 is 47.2 gr. and gives 2900 fps in their 24" test barrel.  This was encouraging, since our 30" Palma barrels will definitely add to that - and we'll need a bit more for 1000 yards.  I got a new 8 lb. jug of IMR 4320, 2009 production, and tested it in the .308 with 155 Sierras.

My first step was to load a few rounds with some moderate charges just to get a feel for the powder.  I loaded and shot 44.0, 45.0 and 46.0 at 200 yards.  You should note that 4320 has very small granules and throws very consistently from the powder measure - in fact it is very much like Reloder 15 in granule size - so for those who prefer to throw, rather than weigh, charges it's a good choice. 

The 44.0 load wasn't too promising, the 45.0 looked fantastic and the 46.0 was also very good.  I judged these on the amount of vertical dispersion in the groups, all of which were fired prone, iron sights from my RPA Quadlock Palma rifle.  Apart from the powder, the rest of the components were: Winchester brass, PMC primers (Russian), and moly-coated Sierra 155 Palma bullets.  All loads in this article were fired with moly-coated bullets and should be reduced by at least 1.0 gr. for bare bullets.

The second stage of the project was to chronograph some loads and shoot on paper with anything that looked interesting (I don't shoot on paper while chronographing).  Unfortunately, I only had a 100 yard range available for this part.  Below is a summary of the chrono testing.  The first three charges were 8 shots each, the remainder 5 shots each.  This was simply to get an idea of how 4320 responds to increases and what sort of velocity range it would give.  Standard Deviation, which is a great measure of a load's uniformity is much more meaningful with 10 to 20 shots per load, so I don't place a huge value on these numbers.  However, the MV showed a fairly normal progression as the charge weight increased.

44.0 gr. - Avg. MV 2762 fps; ES 54; SD 19
45.0 gr. - Avg. MV 2844 fps; ES 37; SD 11
46.0 gr. - Avg. MV 2911 fps; ES 45; SD 13
46.5 gr. - Avg. MV 2952 fps; ES 53; SD 23
47.0 gr. - Avg. MV 2990 fps; ES 18; SD 08
47.5 gr. - Avg. MV 3011 fps; ES 22; SD 08  ***  Exceeds published maximum load.
48.0 gr. - Avg. MV 3048 fps; ES 31; SD 12  *** Exceeds published maximum load.

I stopped at 48.0 grains because that was more than enough velocity for shooting at 1000 yards, the SD was beginning to increase and the powder was now up to the base of the neck (much lower than Varget or 4895 are for comparable velocities); 47.0 gr. is just below the base of the neck.  Also, since the Sierra book shows a maximum load of 47.2 for bare bullets I figured 48.0 was pretty close to maximum pressure with moly although neither the bolt opening, nor extraction, nor the primers showed any signs of undue pressure.

I loaded 13 rounds of the 47.0 gr. load, used three to sight in and fired 10 for record at 100 yards on the NRA 100 yard Smallbore target (2" 10 ring, 1" X ring).  I shot a 100-9X with the only 10 being a high shot that I called high just as I fired it.

My usual .308 Palma load fires the 155's at 3030 fps with an SD in the low single digits.  I've often thought I'm pushing things too hard and plan to back that load down a bit this coming season.  However, it's a very accurate load, so that's not something that I'll do without some testing at 1000 yards.  With that load as a benchmark, I plan to shoot the IMR 4320 loads tomorrow (Sunday Oct. 11) at a 500 yard match.  I'll begin with the current load (which uses H4895) then switch to the 45.0 IMR 4320 load and then the 47.0 IMR 4320 load.  I will clean the barrel a bit after the first string to avoid the possibility of the H4895 fouling making the first few shots of 4320 behave erratically.  I can't say that it would, but I can't say that it won't, so I'll clean.

More tomorrow...

Sunday, October 10, 2009.
I fired the 500 yard prone match today using my RPA Quadlock Palma rifle, Warner sights, Krieger 1:13" twist barrel.  Conditions were calm initially but very windy and switchy by the third string.  Temperature went from high 60's to the low 80's.  I decided to shoot the 4320 loads first and leave the 4895 for last, and that was a fortunate decision as things turned out.  As in the chrono tests, all loads were with moly-coated Sierra 155 Palma bullets, Winchester brass and PMC primers.  Powder charges should be reduced at least 1.0 gr. for bare bullets. 

The load with 45.0 gr. of IMR 4320 gave me a 199-12X with the single 9 being a high shot just off the 10 ring line at 12:00.  Of the 20 shots, three were above the X ring and three below it, and I made a few small elevation changes during the string.  Overall, I would rate that load as good, very useable for the intended purpose of 500 or 600 yard shooting (it's 2844 fps) but no better than my 4895 load for that purpose. 

Next I fired the 47.0 load and that was an eye opener.  The score was 200-15X in conditions that were windier than those in which the first load was fired.  Elevation was noticeably tighter and shots were exactly on call.  This load gives 2990 fps, so it has real potential as a Palma load.  While no 500 yard test can ensure results at 1000, given the MV and reasonably low SD of this load as well as the good performance at 500, I won't hesitate to shoot it at 1000 at the first opportunity.

My current Palma load of 45.5 H4895 was the last one fired and conditions were downright tricky by then.  I shot a 196-12X with the 9's coming on hard and fast reversals that just caught me while I was aiming.  The X count and the good elevation that the load held were enough for me to see that it was working well - the 9's were just the breaks of the game.

Overall, I'm very satisfied with what I've seen in these past few days from IMR 4320 and plan to shoot it in some of our upcoming Palma and 1000 yard matches with the 155's.  It appears to be a very useful alternative to some of the harder to get powders.

Update - December 11, 2009The load is working extremely well at 1000 yards.  In the recently completed Arizona Palma State Championship, several high placing competitors were using the 4320 load.  In one case, we loaded ammo for a national team member at my house the night before the match began.  He was at Bruno's buying all the 4320 he could find after that day's shooting.  We got sub X-ring elevation at 1000 yards from several rifles, and that's all I'm looking for in a Palma load.
 

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