There are several problems with shooting an arrow with too weak a spine (not spline... A truck axle has splines, not arrows).
#1. In severe cases an under spined arrow could break and send the shaft through your hand like the pic above.
#2. Broadhead tipped arrow will shoot like crap. It is far far far better to have an over spined arrow when tipped with broadheads than it is to have an under spined arrow.
#3. When you get severely underspined like you are suggesting, even field tipped arrows become difficult to tune.
#4. A severely weak spined arrow will be detrimental to penetration on game animals.
#5. You aren't going to gain enough speed by going to a weaker spined arrow to make a shits worth of difference in trajectory. You will make a huge difference in crappy arrow flight and crappy penetration.
#6. You start out with less kinetic energy and momentum at the bow and the lighter arrow bleeds off speed faster than the heavy arrow and thus bleeds off KE and momentum faster than the heavier arrow.
#7. The heavier arrow will result in a quieter bow with less residual vibration, and be much easier to tune with broadheads than a super fast arrow.
Since you are wanting to use this weak spined arrow to drop the over all weight of the arrow to gain some speed, that will be minimal at best, I'm gonna run a little arrow ballistics program for you.
Here's the real world drop from 20 to 40 yards when sighted in at 20 yards and shoot the same 20 yard pin at 40 yards to figure the drop from 20 to 40 yards. One arrow weighs 400 grains and is shot at 300 fps. The other arrow weighs 500 grains and is shot at 270 fps. These are real numbers that you can expect from a 330 fps IBO bow set at 70 pounds with a 29" draw length.
400 grain arrow
300 fps
20 yards - zeroed
20 - 40 yard drop - 16.2"
500 grain arrow
270 fps
20 yards - zeroed
20 - 40 yard drop - 20.1"
As you can see there is very little real world difference in the drop of these two arrow, only 4.1 inches from 20 - 40 yards, despite there being a full 100 grains difference in weight and 30 fps difference in the two arrows.
The little bit of speed you might gain by going to an under spined arrow will be 10 fps or less and the difference in trajectory about an inch from 20 - 40 yards.
For an extreme example......... going from a Gold tip 7595 to a Gold tip 3555, which is a major drop in spine and quite dangerous, will get you 40 grains less arrow weight. This will result in about 15 fps difference in speed and about 1.5" in trajectory from 20 - 40 yards. Dropping down to a 5575 will get you even less.
So, in conclusion....... You aren't going to gain a darn thing positive. You will gain a lot of negative.
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