Bullet Lenght, Hornady .277 GMX?

slimjim · Posted: Thu Feb 25, 2010 7:53 am Post subject: Bullet Lenght, Hornady .277 GMX?

Anyone know the bullet length of Hornady's new 130gr GMX for the .270? I was thinking of giving them a try but wanted to check their stability factor. The two on-line databases I used didn't have them listed.

slimjim

Bought a box and brought them home. I was surprised to find they are the longest .277 bullet I have seen. Their length is 1.378 inches. I plug this number in to determine the stability factor - 1.14. I was amazed that a manufacturer would market a bullet to the mass market. On a cold day, the stability factor goes down to 1.05. 1.4 is optimum. I may be speaking too soon but I'm not sure I'm even going to load these. These really need a 1:9 or even a 1:8 twist.

Caliber _ 0.277 Inches
Bullet Weight _ 130 Grains
Bullet Length _ 1.378 Inches
Barrel Twist _ 1:10
muzzle velocity _ 3100 fps
Temperature _ 59 degrees Fahrenheit (59 is standard)
Pressure _ 29.92 inches of mercury (29.92 is standard)

Sg = 1.14

Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may gain some accuracy by going to a slower twist barrel.

chambered221

I'm willing to bet ya it'll work just fine in your 1-10 twist !!! (I didn't see any warnings about twist on the Hornady website)

You must keep in mind that these calculators do not factor in the proper specific gravity for these types of bullets and they also seem to have issues with plastic tips and hollow cavities that shift the center of gravity.

slimjim

My problem is that I like to understand so I always challenge and ask questions.

This new Hornady 130gr GMX is the same length (+0.003) as their 150gr SST. Typically, a copper alloy bullet is 18-20% lighter than a lead-core bullet so this is not unexpected. I have derived from the stability formula that for every 5gr of weight decrease, there needs to be a corresponding decrease in length of 0.025" or an increase in 350fps to maintain the same stability. The only .277 bullet I could find that is longer is the Barnes 150gr MRX BT which has a heavy metal core at a length 1.400". Thus of all the .270 bullets on the market, this one is the least stable.

Calculating Bullet RPM — Spin Rates and Stability

In my quest for knowledge and understanding, I'm getting ready to experiment with bullet shapes. Below are my .270 bullets. From left to right

150gr VLD Berger, 130gr Hornady GMX, 130gr Barnes TSX, modified 130gr Barnes TSX to accept a ballistic tip*, modified 130gr Barnes TSX with tip and enhanced boat-tail**, 120gr Barnes Soild, 110gr Barnes TTSX. * weights 125grs after the machining. ** weights 120grs after the machining. I have to change the process as the boat-tail didn't come out right. I still got an approximate weight. My concern has been that the bullets I modify for testing are getting longer and lighter with corresponding decreases in stability and any velocity increase I get will not be enough to compensate.

Maybe I'll just load-up a few and give them a try tomorrow.

slimjim

Got to the range this morning. Frosty fog that lifted just after sunrise and then temperatures climbed into the 50s. I shot two groups. First let me preface that I loaded the 130gr Barnes TSX with 56gr of IMR4350. That is supposed to be max load but I show no signs of high pressure like when I shoot the 110gr Barnes TTSX with 60.5gr of IMR4350 which takes some of the round out of the edge of the primers. I tried the 110s with 60.7grs once and the dimple around the firing pin had a small lip on it you could feel with your finger nail so stayed with 60.5. Based on this experience and this being a copper alloy bullet I loaded 3 rounds with 55gr IMR4350 (COAL 3.325") and an additional 3 rounds with 55.5gr. I shot at 250 yards. The first picture is the results with 55gr. There were no signs of high pressure.

The second picture is with 55.5gr. A little surprised to see one string horizontal and the other vertical. The 55.5gr started to show flattening of the primer but no signs on the firing pin dimple.

What load would you recommend trying next? I could go with 55.3 and see if the vertical stringing was reduced. Maybe try 56 as the top end which would allow me to have a direct compare with the 130gr Barnes TSX if it grouped well enough.

slimjim

It was only 6 shots but I did clean the bore when I returned home and did not find excessive copper fouling.

Elvis

I keep hearing about this stability factor. I had 20 rounds of lapua 160grn round nose for my .270 which is a post 64 mod 70 lightweight 1 in10 twist. the lapuas shot a couple of deer for me but after carting them around for years [about 14] i fired them off into a target to reload the cases. result 1 inch group. these were LONG bullets. they looked like taylor made ciggies. the only rounds I have ever loaded that dont group well eg 3inch groups were 110grn hornady projectiles . they were still deadly medicin on wallabies.

slimjim

Elvis wrote:

I had 20 rounds of lapua 160grn round nose for my .270 ... result 1 inch group. these were LONG bullets.

160gr round nose are short for their weight because they are not pointed, e.g., short for their sectional density and are easier to stabilize. With lengths being equal, heavier bullets stabilize better.

slimjim

I realized this morning, I hadn't sorted the box of bullets by weight. They ranged from 129.4 to 130.8 grains. I may go repeat using the 55gr IMR4350 using a group with the same weight bullets and one with bullets from the weight extremes. The vertical spread could be caused by small velocity differences.

Elvis

thanks for that slimjim. I may just have to try some roundnose 110grn to see how they go. darn that means more shooting.

slimjim

Regarding stability of this GMX bullet, I wrote to Hornady and asked for additional info because I was concerned about its length with a 1:10 twist. I've read information on the web that the Miller Stability Equation is conservative and thought Hornady might have more insight. From Hornady's explaination, the Miller Stability Equation is conservative. Here is information from Hornady.

"We use the PRODAS code for all our aeroballistic modelling and have found it to be exceptionally accurate over the years. It predicts a gyro of 1.3 at standard atmosphere and 1.1 at -15F. We have encountered no complaints in two years with this bullet. In fact all we have gotten is praise for how accurate it is. I agree the gyro is getting a little low but we felt we had to have a 130 gr bullet in .270 to compete and we had prior experience with some of our match bullets that said it would work fine. Bear in mind that the bullet is getting more stable the further it flies as it slows down. I hope this helps you. Don't hesitate to contact me if you have any other questions."

Hornady Manufacturing, Inc

slimjim

When Hornady writes about "praise for accuracy" with their GMX bullet, I must say I experienced this also. After seeing the vertical dispersion in the second group above, I check the remaining 44 bullets in the box for weight variation. Bullets ranged from 129.4 to 130.9 grains. I have found this to be typical of the Barnes bullets I shoot. When I sorted the GMX bullets by weight, loaded three at 130.0gr, and refired with 55.5gr of IMR4350, this was the group I got at 500 yards (double the distance above). The wind was variable, 6 to 10 mph, from behind. The vertical dispersion shrank to 0.25 MOA. This is the best group I've shot with my Tikka T3 Lite SS.

chambered221

Having a tough time trying to decide if ya like this bullet or not ??? Trying to talk yourself into it ???

wtf

Welcome to the world of re-loading !!!

slimjim

chambered221 wrote:

Having a tough time trying to decide if ya like this bullet or not ??? Trying to talk yourself into it?

Your right, it does sound like I can't decide. I've become more aware of bullet stability from the discussions we have had on our forum. With a stability calculation of 1.15 using Miller's equation, I almost boxed them up and took them back. Glad I decided to shoot them. They certainly have shot well, my best group every after only 9 shots! Its hard not to like that. Hornady have been very responsive which is a welcome change. I have 44 more bullets to shoot so have lots more to learn. I may also tinker with them on the lathe and see if I can improve BC or velocity since there is a little more margin there than I first thought.