Well I have a couple dozen of them, If installed correctly (as shown in the installation video)they will hold good inside of the arrow shaft. When I started using these I had a concrete block wall that was shielded by a sheet of plywood and that was my backstop, I was shooting at 19yds with a 55lb Super Grizzly, when I did miss my target I had a very FEW NOCKS blow out the back. When that did happen I would pull them partially out and crimp them again in a few more spots and I was good.As for the weight tubes affecting spine, IF THEY DID it was minimal. I did some experimenting with my 65lb Super kodiak shooting arrows with and without the weight tubes at greater distances like 50yds plus. The only difference I could actuallt notice was arrow speed. The weight tubes in question are manufactured much like a common straw only slightly thicker to add weight. The tubes do not carry any structural enhancements to the arrow. I am not an Engineer but I have done some indepth research with these. If you are looking to add some overall weight to your arrows, figure out how much overall weight you are trying to gain and order some accordingly, BTW: do yourself a favor and watch the installation video that 3Rivers put together and follow it exactly. I am sure you will be suprised, OHH Yeh! I use GTs exclusively anmd will continue to use them.