Pressure differences

[cdoc42]

I’m working up a load with a Hornady ELD-X 145gr bullet in .270 Winchester. For the past 45 years I’ve loaded 150gr Hornady Spire with 56.0gr of H-4831 w/o any issues.
I compared the Hodgdon data for 140gr SFT SP and 150gr HDY SP with H1000, H4831 and IMR 7828.
My 150gr load is at max and assuming I get the same velocity the pressure is 51,200 CUP which is greater than any of the 5 other loads. So I feel comfortable loading the 145gr bullet with the 150gr data as a basis and starting 2 grains below max with each of the 3 powders because I should be under the max pressure seen with the 150gr bullet. Here is the data from the Hodgdon web site for a 150gr HDY SP:

H4831 Max 55.7gr (I use 56.0) Vel 2804 CUP: 51,200 My thinking is I should be safe starting with 54.0gr with the 145gr ELD-X and creeping up if accuracy is not satisfactory. But what about chamber pressure because of the difference in bullet shapes? So I asked A.I., but I neglected to specify chamber pressure. The reply follows:

I asked A.I. if I use the same powder and charge with a 140gr SP and a 140gr ELD will there be a significant difference in pressure?

"According to the web search results, the Hornady ELD 140grains bullet will produce more CUP or PSI than the Hornady 140grains BTSP, using the same powder and charge. This is because the ELD bullet has a higher ballistic coefficient and a longer ogive than the BTSP bullet, which means it has less drag and more resistance to air pressure12. The ELD bullet also has a Heat Shield Tip that prevents deformation from aerodynamic heating, which improves its stability and accuracy3. The BTSP bullet, on the other hand, has a soft point that expands on impact, which reduces its penetration and increases its drag4. Therefore, the ELD bullet will have a higher pressure curve than the BTSP bullet, using the same powder and charge."

Unless I have it all wrong, I am looking for chamber pressure before the bullet exits the rifle, which is not the same as the air pressure in flight discussed by A.I.

Comments?

[Shadow9mm]

According to the hornady 11th edition
270 win
145g eldx
Hornady brass
Winchester large rifle primers
Col 3.330
Max for H-4831 is listed as 59.9g
This should produce 3000fps out of a 24in barrel.

Hornady lists 56.5g with a velocity of 2800fps
And 54.7g at 2700fps

On a side note, hornady 11th lists a max of 57.3 of h-4831 with their 150g interlock (spire point). Being 1.3g under max with that load, never having issues makes a fair bit of sense.

[ballardw]

Longer bullet with loaded within the same magazine length limit could mean more of the space in the case is used by the bullet. Less air space in the case for the same powder charge generally would increase pressure in chamber.

[cdoc42]

Thanx, Shadow, I didn't have access to Hornady info. Is there a recipe using H-1000?
I ask because I used 56.0gr with a 117gr Sierra in 25-06 and got a 0.4" four-shot group and later, 0.2" group. I thought it was unusual to use a slow powder with a light bullet in that caliber but what do I know anyway?
Your info reveals very clearly how the data available depends on how closely your inventory is to that used by the manufacturer, raising the alert level as one proceeds in this hobby.

[cdoc42]

ballardw, that introduces yet another variable - the magazine length, so the results reported depend on seating to reference book advice vs seating to whatever length produces the best accuracy. Good thought.

[Shadow9mm]

Originally Posted by cdoc42

Thanx, Shadow, I didn't have access to Hornady info. Is there a recipe using H-1000?
I ask because I used 56.0gr with a 117gr Sierra in 25-06 and got a 0.4" four-shot group and later, 0.2" group. I thought it was unusual to use a slow powder with a light bullet in that caliber but what do I know anyway?
Your info reveals very clearly how the data available depends on how closely your inventory is to that used by the manufacturer, raising the alert level as one proceeds in this hobby.

Unfortunately neither my 10th or 11th edition hornady manuals list H-1000 for 270win.