Some schemes create paper trails in federal agencies, others do not. In America you can acquire rifles without filling out any paperwork at all, let alone lying to a federal agency on paper. Converting those rifles to automatics can, again, be done without lying on any paperwork in a variety of ways (some more effective than others.) Somebody up to no good would be better served by low-profile acquisition schemes that fly under the radar of regulators, rather than getting their attention then trying to actively deceive them.

All schemes may be risky, but they're not all equally risky. Some schemes are more risky than others. However all of these schemes probably have negative expected payouts if you factor in the FBI being pretty damn good at their jobs. Whatever crime you hypothetically plan on committing with automatic rifles will almost certainly not have a positive payoff when you include the cost of getting busted, and you almost certainly will eventually get busted. When smart people decide to be criminals, they choose white collar crime (arrest rates are very low, and sentencing for those captured is frequently lax.) Violent crime is for idiots who fail to rationally consider the likely consequences of their actions.